这篇技术博客是关于动力节点MySQL教程做的笔记总结，方便大家的学习同时也方便自己。该技术博客会配合一个sql脚本文件来学习，sql脚本文件名为：bjpowernode.sql，代码如下：
DROP TABLE IF EXISTS EMP;DROP TABLE IF EXISTS DEPT;DROP TABLE IF EXISTS SALGRADE;CREATE TABLE DEPT (DEPTNO int(2) not null ,DNAME VARCHAR(14) ,LOC VARCHAR(13),primary key (DEPTNO));CREATE TABLE EMP (EMPNO int(4) not null ,ENAME VARCHAR(10),JOB VARCHAR(9),MGR INT(4),HIREDATE DATE DEFAULT NULL,SAL DOUBLE(7,2),COMM DOUBLE(7,2),primary key (EMPNO),DEPTNO INT(2) );CREATE TABLE SALGRADE ( GRADE INT,LOSAL INT,HISAL INT );INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES ( 10, 'ACCOUNTING', 'NEW YORK'); INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES ( 20, 'RESEARCH', 'DALLAS'); INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES ( 30, 'SALES', 'CHICAGO'); INSERT INTO DEPT ( DEPTNO, DNAME, LOC ) VALUES ( 40, 'OPERATIONS', 'BOSTON'); commit; INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800, NULL, 20); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600, 300, 30); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250, 500, 30); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975, NULL, 20); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250, 1400, 30); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850, NULL, 30); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450, NULL, 10); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000, NULL, 20); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7839, 'KING', 'PRESIDENT', NULL, '1981-11-17', 5000, NULL, 10); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7844, 'TURNER', 'SALESMAN', 7698, '1981-09-08', 1500, 0, 30); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7876, 'ADAMS', 'CLERK', 7788, '1987-05-23', 1100, NULL, 20); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950, NULL, 30); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000, NULL, 20); INSERT INTO EMP ( EMPNO, ENAME, JOB, MGR, HIREDATE, SAL, COMM,DEPTNO ) VALUES ( 7934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300, NULL, 10); commit; INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES ( 1, 700, 1200); INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES ( 2, 1201, 1400); INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES ( 3, 1401, 2000); INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES ( 4, 2001, 3000); INSERT INTO SALGRADE ( GRADE, LOSAL, HISAL ) VALUES ( 5, 3001, 9999); commit;DB: DataBase（数据库，数据库实际上在硬盘上以文件的形式存在）DBMS: DataBase Management System（数据库管理系统，常见的有：MySQL Oracle DB2 Sybase SqlServer...）SQL: SQL属于高级语言（结构化查询语言），是一门标准通用的语言。标准的sql适合于所有的数据库产品。SQL语句在执行的时候，实际上内部也会先进行编译，然后再执行sql。（sql语句的编译由DBMS完成。）DBMS负责执行sql语句，通过执行sql语句来操作DB当中的数据。DBMS -(执行)-> SQL -(操作)-> DB表：table是数据库的基本组成单元，所有的数据都以表格的形式组织，目的是可读性强。一个表包括行和列：行：被称为数据/记录(data)列：被称为字段(column)，类似于java类中的属性学号(int)姓名(varchar)年龄(int)------------------------------------110张三20120李四21每一个字段应该包括哪些属性？字段名、数据类型、相关的约束。DQL（数据查询语言）: 查询语句，凡是select语句都是DQL。DML（数据操作语言）：insert delete update，对表当中的数据进行增删改。DDL（数据定义语言）：create drop alter，对表结构的增删改。（对数据库同理）TCL（事务控制语言）：commit提交事务，rollback回滚事务。(TCL中的T是Transaction)DCL（数据控制语言）: grant授权、revoke撤销权限等。第一步：登录mysql数据库管理系统，输入数据库账号密码dos命令窗口：mysql -uroot -proot第二步：查看有哪些数据库show databases; (这个不是SQL语句，属于MySQL的命令。)+--------------------+| Database |+--------------------+| information_schema || mysql || performance_schema || test |+--------------------+第三步：创建属于我们自己的数据库create database bjpowernode; (这个不是SQL语句，属于MySQL的命令。)第四步：使用bjpowernode数据use bjpowernode; (这个不是SQL语句，属于MySQL的命令。)第五步：查看当前使用的数据库中有哪些表？show tables; (这个不是SQL语句，属于MySQL的命令。)第六步：初始化数据mysql> source D:\course\05-MySQL\resources\bjpowernode.sql（该文件可以添加微信领取）注意：数据初始化完成之后，有三张表：+-----------------------+| Tables_in_bjpowernode |+-----------------------+| dept || emp || salgrade |+-----------------------+bjpowernode.sql，这个文件以sql结尾，这样的文件被称为“sql脚本”当一个文件的扩展名是.sql，并且该文件中编写了大量的sql语句，我们称这样的文件为sql脚本。注意：直接使用source命令可以执行sql脚本。sql脚本中的数据量太大的时候，无法打开，请使用source命令完成初始化。+-----------------------+| Tables_in_bjpowernode |+-----------------------+| dept | (部门表)| emp | (员工表)| salgrade | (工资等级表)+-----------------------+mysql> desc dept;+--------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+--------+-------------+------+-----+---------+-------+| DEPTNO | int(2) | NO | PRI | NULL | |部门编号| DNAME | varchar(14) | YES | | NULL | |部门名称| LOC | varchar(13) | YES | | NULL | |部门位置+--------+-------------+------+-----+---------+-------+mysql> desc emp;+----------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+----------+-------------+------+-----+---------+-------+| EMPNO | int(4) | NO | PRI | NULL | |员工编号| ENAME | varchar(10) | YES | | NULL | |员工姓名| JOB | varchar(9) | YES | | NULL | |工作岗位| MGR | int(4) | YES | | NULL | |上级领导编号| HIREDATE | date | YES | | NULL | |入职日期| SAL | double(7,2) | YES | | NULL | |月薪| COMM | double(7,2) | YES | | NULL | |补助/津贴| DEPTNO | int(2) | YES | | NULL | |部门编号+----------+-------------+------+-----+---------+-------+mysql> desc salgrade;+-------+---------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| GRADE | int(11) | YES | | NULL | |等级| LOSAL | int(11) | YES | | NULL | |最低薪资| HISAL | int(11) | YES | | NULL | |最高薪资+-------+---------+------+-----+---------+-------+mysql> select * from emp;+-------+--------+-----------+------+------------+---------+---------+--------+| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |+-------+--------+-----------+------+------------+---------+---------+--------+| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 || 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 || 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 || 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 || 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 || 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 || 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 || 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 || 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 || 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 || 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 || 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 || 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 || 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |+-------+--------+-----------+------+------------+---------+---------+--------+mysql> select * from dept;+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+mysql> select * from salgrade;+-------+-------+-------+| GRADE | LOSAL | HISAL |+-------+-------+-------+| 1 | 700 | 1200 || 2 | 1201 | 1400 || 3 | 1401 | 2000 || 4 | 2001 | 3000 || 5 | 3001 | 9999 |+-------+-------+-------+mysql> select database(); 查看当前使用的是哪个数据库+-------------+| database() |+-------------+| bjpowernode |+-------------+mysql> select version(); 查看mysql的版本号。+-----------+| version() |+-----------+| 5.5.36 |+-----------+mysql> show create table 表名； 查看创建表的语句\c 命令，结束一条语句。exit 命令，退出mysql。语法格式：select 字段名1,字段名2,字段名3,.... from 表名;提示：1、任何一条sql语句以“;”结尾。2、sql语句不区分大小写。查询员工的年薪？（字段可以参与数学运算。）select ename,sal * 12 from emp;+--------+----------+| ename | sal * 12 |+--------+----------+| SMITH | 9600.00 || ALLEN | 19200.00 || WARD | 15000.00 || JONES | 35700.00 || MARTIN | 15000.00 || BLAKE | 34200.00 || CLARK | 29400.00 || SCOTT | 36000.00 || KING | 60000.00 || TURNER | 18000.00 || ADAMS | 13200.00 || JAMES | 11400.00 || FORD | 36000.00 || MILLER | 15600.00 |+--------+----------+给查询结果的列重命名？select ename,sal * 12 as yearsal from emp;别名中有中文？select ename,sal * 12 as 年薪 from emp; // 错误select ename,sal * 12 as '年薪' from emp;+--------+----------+| ename | 年薪 |+--------+----------+| SMITH | 9600.00 || ALLEN | 19200.00 || WARD | 15000.00 || JONES | 35700.00 || MARTIN | 15000.00 || BLAKE | 34200.00 || CLARK | 29400.00 || SCOTT | 36000.00 || KING | 60000.00 || TURNER | 18000.00 || ADAMS | 13200.00 || JAMES | 11400.00 || FORD | 36000.00 || MILLER | 15600.00 |+--------+----------+注意：标准sql语句中要求字符串使用单引号括起来。虽然mysql支持双引号，尽量别用。as关键字可以省略？mysql> select empno,ename,sal * 12 yearsal from emp;+-------+--------+----------+| empno | ename | yearsal |+-------+--------+----------+| 7369 | SMITH | 9600.00 || 7499 | ALLEN | 19200.00 || 7521 | WARD | 15000.00 || 7566 | JONES | 35700.00 || 7654 | MARTIN | 15000.00 || 7698 | BLAKE | 34200.00 || 7782 | CLARK | 29400.00 || 7788 | SCOTT | 36000.00 || 7839 | KING | 60000.00 || 7844 | TURNER | 18000.00 || 7876 | ADAMS | 13200.00 || 7900 | JAMES | 11400.00 || 7902 | FORD | 36000.00 || 7934 | MILLER | 15600.00 |+-------+--------+----------+查询所有字段？select * from emp; // 实际开发中不建议使用*，效率较低。语法格式：select 字段,字段...from表名where条件;执行顺序：先from，然后where，最后select查询工资等于5000的员工姓名？select ename from emp where sal = 5000;+-------+| ename |+-------+| KING |+-------+查询SMITH的工资？select sal from emp where ename = 'SMITH'; // 字符串使用单引号括起来。+--------+| sal |+--------+| 800.00 |+--------+找出工资高于3000的员工？select ename,sal from emp where sal > 3000;select ename,sal from emp where sal >= 3000;select ename,sal from emp where sal < 3000;select ename,sal from emp where sal <= 3000;找出工资不等于3000的？select ename,sal from emp where sal <> 3000;select ename,sal from emp where sal != 3000;找出工资在1100和3000之间的员工，包括1100和3000？select ename,sal from emp where sal >= 1100 and sal <= 3000;select ename,sal from emp where sal between 1100 and 3000; // between...and...是闭区间 [1100 ~ 3000]select ename,sal from emp where sal between 3000 and 1100; // 查询不到任何数据between and在使用的时候必须左小右大。between and除了可以使用在数字方面之外，还可以使用在字符串方面。select ename from emp where ename between 'A' and 'C';+-------+| ename |+-------+| ALLEN || BLAKE || ADAMS |+-------+select ename from emp where ename between 'A' and 'D'; // 左闭右开。找出哪些人津贴为NULL？在数据库当中NULL不是一个值，代表什么也没有，为空。空不是一个值，不能用等号衡量。必须使用 is null或者is not nullselect ename,sal,comm from emp where comm is null;+--------+---------+------+| ename | sal | comm |+--------+---------+------+| SMITH | 800.00 | NULL || JONES | 2975.00 | NULL || BLAKE | 2850.00 | NULL || CLARK | 2450.00 | NULL || SCOTT | 3000.00 | NULL || KING | 5000.00 | NULL || ADAMS | 1100.00 | NULL || JAMES | 950.00 | NULL || FORD | 3000.00 | NULL || MILLER | 1300.00 | NULL |+--------+---------+------+select ename,sal,comm from emp where comm = null;Empty set (0.00 sec)找出哪些人津贴不为NULL？select ename,sal,comm from emp where comm is not null;+--------+---------+---------+| ename | sal | comm |+--------+---------+---------+| ALLEN | 1600.00 | 300.00 || WARD | 1250.00 | 500.00 || MARTIN | 1250.00 | 1400.00 || TURNER | 1500.00 | 0.00 |+--------+---------+---------+找出哪些人没有津贴？select ename,sal,comm from emp where comm is null or comm = 0;+--------+---------+------+| ename | sal | comm |+--------+---------+------+| SMITH | 800.00 | NULL || JONES | 2975.00 | NULL || BLAKE | 2850.00 | NULL || CLARK | 2450.00 | NULL || SCOTT | 3000.00 | NULL || KING | 5000.00 | NULL || TURNER | 1500.00 | 0.00 || ADAMS | 1100.00 | NULL || JAMES | 950.00 | NULL || FORD | 3000.00 | NULL || MILLER | 1300.00 | NULL |+--------+---------+------+找出工作岗位是MANAGER和SALESMAN的员工？select ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';+--------+----------+| ename | job |+--------+----------+| ALLEN | SALESMAN || WARD | SALESMAN || JONES | MANAGER || MARTIN | SALESMAN || BLAKE | MANAGER || CLARK | MANAGER || TURNER | SALESMAN |+--------+----------+and和or联合起来用：找出薪资大于1000的并且部门编号是20或30部门的员工。select ename,sal,deptno from emp where sal > 1000 and deptno = 20 or deptno = 30; // 错误的select ename,sal,deptno from emp where sal > 1000 and (deptno = 20 or deptno = 30); // 正确的。注意：当运算符的优先级不确定的时候加小括号。in等同于or：找出工作岗位是MANAGER和SALESMAN的员工？select ename,job from emp where job = 'SALESMAN' or job = 'MANAGER';select ename,job from emp where job in('SALESMAN', 'MANAGER');select ename,job from emp where sal in(800, 5000); // in后面的值不是区间，是具体的值。+-------+-----------+| ename | job |+-------+-----------+| SMITH | CLERK || KING | PRESIDENT |+-------+-----------+not in: 不在这几个值当中。select ename,job from emp where sal not in(800, 5000);模糊查询like ? 找出名字当中含有O的？（在模糊查询当中，必须掌握两个特殊的符号，一个是%，一个是_）%代表任意多个字符，_代表任意1个字符。select ename from emp where ename like '%O%';+-------+| ename |+-------+| JONES || SCOTT || FORD |+-------+找出名字中第二个字母是A的？select ename from emp where ename like '_A%';+--------+| ename |+--------+| WARD || MARTIN || JAMES |+--------+找出名字中有下划线的？mysql> select * from t_user;+------+----------+| id | name |+------+----------+| 1 | zhangsan || 2 | lisi || 3 | WANG_WU |+------+----------+select name from t_user where name like '%_%';+----------+| name |+----------+| zhangsan || lisi || WANG_WU |+----------+select name from t_user where name like '%\_%';+---------+| name |+---------+| WANG_WU |+---------+找出名字中最后一个字母是T的？select ename from emp where ename like '%T';+-------+| ename |+-------+| SCOTT | +-------+按照工资升序，找出员工名和薪资？select ename,sal from emp order bysal;+--------+---------+| ename | sal |+--------+---------+| SMITH | 800.00 || JAMES | 950.00 || ADAMS | 1100.00 || WARD | 1250.00 || MARTIN | 1250.00 || MILLER | 1300.00 || TURNER | 1500.00 || ALLEN | 1600.00 || CLARK | 2450.00 || BLAKE | 2850.00 || JONES | 2975.00 || FORD | 3000.00 || SCOTT | 3000.00 || KING | 5000.00 |+--------+---------+注意：默认是升序。怎么指定升序或者降序呢？asc表示升序，desc表示降序。select ename , sal from emp order by sal; // 升序select ename , sal from emp order by sal asc; // 升序select ename , sal from emp order by sal desc; // 降序。按照工资的降序排列，当工资相同的时候再按照名字的升序排列。select ename,sal from emp order by sal desc;select ename,sal from emp order by sal desc , ename asc;注意：越靠前的字段越能起到主导作用。只有当前面的字段无法完成排序的时候，才会启用后面的字段。找出工作岗位是SALESMAN的员工，并且要求按照薪资的降序排列。select ename,job,salfromempwhere job = 'SALESMAN'order bysal desc;+--------+----------+---------+| ename | job | sal |+--------+----------+---------+| ALLEN | SALESMAN | 1600.00 || TURNER | SALESMAN | 1500.00 || WARD | SALESMAN | 1250.00 || MARTIN | SALESMAN | 1250.00 |+--------+----------+---------+select 字段3from表名1where条件2order by....4order by是最后执行的。count 计数sum 求和avg 平均值max 最大值min 最小值记住：所有的分组函数都是对“某一组”数据进行操作的。找出工资总和？select sum(sal) from emp;找出最高工资？select max(sal) from emp;找出最低工资？select min(sal) from emp;找出平均工资？select avg(sal) from emp;找出总人数？select count(*) from emp;select count(ename) from emp;分组函数一共5个。分组函数还有另一个名字：多行处理函数。多行处理函数的特点：输入多行，最终输出的结果是1行。分组函数自动忽略NULL。select count(comm) from emp;+-------------+| count(comm) |+-------------+| 4 |+-------------+select sum(comm) from emp;+-----------+| sum(comm) |+-----------+| 2200.00 |+-----------+select sum(comm) from emp where comm is not null; // 不需要额外添加这个过滤条件。sum函数自动忽略NULL。找出工资高于平均工资的员工？select avg(sal) from emp; // 平均工资+-------------+| avg(sal) |+-------------+| 2073.214286 |+-------------+select ename,sal from emp where sal > avg(sal); //ERROR 1111 (HY000): Invalid use of group function思考以上的错误信息：无效的使用了分组函数？原因：SQL语句当中有一个语法规则，分组函数不可直接使用在where子句当中。why????怎么解释？因为group by是在where执行之后才会执行的。select5..from1..where2..group by3..having4..order by6..count(*)和count(具体的某个字段)，他们有什么区别？count(*):不是统计某个字段中数据的个数，而是统计总记录条数。（和某个字段无关）count(comm): 表示统计comm字段中不为NULL的数据总数量。分组函数也能组合起来用：select count(*),sum(sal),avg(sal),max(sal),min(sal) from emp;+----------+----------+-------------+----------+----------+| count(*) | sum(sal) | avg(sal) | max(sal) | min(sal) |+----------+----------+-------------+----------+----------+| 14 | 29025.00 | 2073.214286 | 5000.00 | 800.00 |+----------+----------+-------------+----------+----------+找出工资高于平均工资的员工？第一步：找出平均工资select avg(sal) from emp;+-------------+| avg(sal) |+-------------+| 2073.214286 |+-------------+第二步：找出高于平均工资的员工select ename,sal from emp where sal > 2073.214286;+-------+---------+| ename | sal |+-------+---------+| JONES | 2975.00 || BLAKE | 2850.00 || CLARK | 2450.00 || SCOTT | 3000.00 || KING | 5000.00 || FORD | 3000.00 |+-------+---------+select ename,sal from emp where sal > (select avg(sal) from emp);什么是单行处理函数？输入一行，输出一行。计算每个员工的年薪？select ename,(sal+comm)*12 as yearsal from emp;重点：所有数据库都是这样规定的，只要有NULL参与的运算结果一定是NULL。使用ifnull函数：select ename,(sal+ifnull(comm,0))*12 as yearsal from emp;ifnull() 空处理函数？ifnull(可能为NULL的数据,被当做什么处理) ： 属于单行处理函数。select ename,ifnull(comm,0) as comm from emp;+--------+---------+| ename | comm |+--------+---------+| SMITH | 0.00 || ALLEN | 300.00 || WARD | 500.00 || JONES | 0.00 || MARTIN | 1400.00 || BLAKE | 0.00 || CLARK | 0.00 || SCOTT | 0.00 || KING | 0.00 || TURNER | 0.00 || ADAMS | 0.00 || JAMES | 0.00 || FORD | 0.00 || MILLER | 0.00 |+--------+---------+group by ： 按照某个字段或者某些字段进行分组。having : having是对分组之后的数据进行再次过滤。案例：找出每个工作岗位的最高薪资。select max(sal),job from emp group by job;+----------+-----------+| max(sal) | job |+----------+-----------+| 3000.00 | ANALYST || 1300.00 | CLERK || 2975.00 | MANAGER || 5000.00 | PRESIDENT || 1600.00 | SALESMAN |+----------+-----------+注意：分组函数一般都会和group by联合使用，这也是为什么它被称为分组函数的原因。并且任何一个分组函数（count sum avg max min）都是在group by语句执行结束之后才会执行的。当一条sql语句没有group by的话，整张表的数据会自成一组。select ename,max(sal),job from emp group by job;以上在mysql当中，查询结果是有的，但是结果没有意义，在Oracle数据库当中会报错。语法错误。Oracle的语法规则比MySQL语法规则严谨。记住一个规则：当一条语句中有group by的话，select后面只能跟分组函数和参与分组的字段。每个工作岗位的平均薪资？select job,avg(sal) from emp group by job;+-----------+-------------+| job | avg(sal) |+-----------+-------------+| ANALYST | 3000.000000 || CLERK | 1037.500000 || MANAGER | 2758.333333 || PRESIDENT | 5000.000000 || SALESMAN | 1400.000000 |+-----------+-------------+多个字段能不能联合起来一块分组？案例：找出每个部门不同工作岗位的最高薪资。select deptno,job,max(sal)fromempgroup bydeptno,job;找出每个部门的最高薪资，要求显示薪资大于2900的数据。第一步：找出每个部门的最高薪资select max(sal),deptno from emp group by deptno;+----------+--------+| max(sal) | deptno |+----------+--------+| 5000.00 | 10 || 3000.00 | 20 || 2850.00 | 30 |+----------+--------+第二步：找出薪资大于2900select max(sal),deptno from emp group by deptno having max(sal) > 2900; // 这种方式效率低。+----------+--------+| max(sal) | deptno |+----------+--------+| 5000.00 | 10 || 3000.00 | 20 |+----------+--------+select max(sal),deptno from emp where sal > 2900 group by deptno; // 效率较高，建议能够使用where过滤的尽量使用where。+----------+--------+| max(sal) | deptno |+----------+--------+| 5000.00 | 10 || 3000.00 | 20 |+----------+--------+找出每个部门的平均薪资，要求显示薪资大于2000的数据。第一步：找出每个部门的平均薪资select deptno,avg(sal) from emp group by deptno;+--------+-------------+| deptno | avg(sal) |+--------+-------------+| 10 | 2916.666667 || 20 | 2175.000000 || 30 | 1566.666667 |+--------+-------------+第二步：要求显示薪资大于2000的数据select deptno,avg(sal) from emp group by deptno having avg(sal) > 2000;+--------+-------------+| deptno | avg(sal) |+--------+-------------+| 10 | 2916.666667 || 20 | 2175.000000 |+--------+-------------+where后面不能使用分组函数：select deptno,avg(sal) from emp where avg(sal) > 2000 group by deptno;// 错误了。这种情况只能使用having过滤。select 5..from1..where2..group by3..having 4..order by6..mysql> select distinct job from emp; // distinct关键字去除重复记录。+-----------+| job |+-----------+| CLERK || SALESMAN || MANAGER || ANALYST || PRESIDENT |+-----------+mysql> select ename,distinct job from emp;以上的sql语句是错误的。记住：distinct只能出现在所有字段的最前面。mysql> select distinct deptno,job from emp;+--------+-----------+| deptno | job |+--------+-----------+| 20 | CLERK || 30 | SALESMAN || 20 | MANAGER || 30 | MANAGER || 10 | MANAGER || 20 | ANALYST || 10 | PRESIDENT || 30 | CLERK || 10 | CLERK |+--------+-----------+案例：统计岗位的数量？select count(distinct job) from emp;+---------------------+| count(distinct job) |+---------------------+| 5 |+---------------------+在实际开发中，大部分的情况下都不是从单表中查询数据，一般都是多张表联合查询取出最终的结果。在实际开发中，一般一个业务都会对应多张表，比如：学生和班级，起码两张表。stunostunameclassnoclassname-----------------------------------------------------------------------------------1zs1北京大兴区亦庄经济技术开发区第二中学高三1班2ls1北京大兴区亦庄经济技术开发区第二中学高三1班...学生和班级信息存储到一张表中，结果就像上面一样，数据会存在大量的重复，导致数据的冗余。根据语法出现的年代来划分的话，包括：SQL92（一些老的DBA可能还在使用这种语法。DBA：DataBase Administrator，数据库管理员）SQL99（比较新的语法）根据表的连接方式来划分，包括：内连接：等值连接非等值连接自连接外连接：左外连接（左连接）右外连接（右连接）全连接（这个不讲，很少用！）笛卡尔积现象：是连接查询的一种现象案例：找出每一个员工的部门名称，要求显示员工名和部门名。EMP表+--------+--------+| ename | deptno |+--------+--------+| SMITH | 20 || ALLEN | 30 || WARD | 30 || JONES | 20 || MARTIN | 30 || BLAKE | 30 || CLARK | 10 || SCOTT | 20 || KING | 10 || TURNER | 30 || ADAMS | 20 || JAMES | 30 || FORD | 20 || MILLER | 10 |+--------+--------+DEPT表+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+select ename,dname from emp,dept;+--------+------------+| ename | dname |+--------+------------+| SMITH | ACCOUNTING || SMITH | RESEARCH || SMITH | SALES || SMITH | OPERATIONS || ALLEN | ACCOUNTING || ALLEN | RESEARCH || ALLEN | SALES || ALLEN | OPERATIONS |............56 rows in set (0.00 sec)笛卡尔积现象：当两张表进行连接查询的时候，没有任何条件进行限制，最终的查询结果条数是两张表记录条数的乘积。关于表的别名：select e.ename,d.dname from emp e,dept d;表的别名有什么好处？第一：执行效率高。第二：可读性好。当然是加条件进行过滤思考：避免了笛卡尔积现象，会减少记录的匹配次数吗？不会，次数还是56次。只不过显示的是有效记录。案例：找出每一个员工的部门名称，要求显示员工名和部门名。selecte.ename,d.dnamefromemp e , dept dwheree.deptno = d.deptno; //SQL92，以后不用。+--------+------------+| ename | dname |+--------+------------+| CLARK | ACCOUNTING || KING | ACCOUNTING || MILLER | ACCOUNTING || SMITH | RESEARCH || JONES | RESEARCH || SCOTT | RESEARCH || ADAMS | RESEARCH || FORD | RESEARCH || ALLEN | SALES || WARD | SALES || MARTIN | SALES || BLAKE | SALES || TURNER | SALES || JAMES | SALES |+--------+------------+最大特点是：条件是等量关系。案例：查询每个员工的部门名称，要求显示员工名和部门名。SQL92:（太老，不用了）select e.ename,d.dnamefromemp e, dept dwheree.deptno = d.deptno;SQL99：（常用的）select e.ename,d.dnamefromemp ejoindept done.deptno = d.deptno;// inner可以省略的，带着inner目的是可读性好一些。select e.ename,d.dnamefromemp einner joindept done.deptno = d.deptno;语法：...AjoinBon连接条件where...SQL99语法结构更清晰一些：表的连接条件和后来的where条件分离了。+--------+------------+| ename | dname |+--------+------------+| CLARK | ACCOUNTING || KING | ACCOUNTING || MILLER | ACCOUNTING || SMITH | RESEARCH || JONES | RESEARCH || SCOTT | RESEARCH || ADAMS | RESEARCH || FORD | RESEARCH || ALLEN | SALES || WARD | SALES || MARTIN | SALES || BLAKE | SALES || TURNER | SALES || JAMES | SALES |+--------+------------+最大的特点是：连接条件中的关系是非等量关系。案例：找出每个员工的工资等级，要求显示员工名、工资、工资等级。mysql> select ename,sal from emp; e+--------+---------+| ename | sal |+--------+---------+| SMITH | 800.00 || ALLEN | 1600.00 || WARD | 1250.00 || JONES | 2975.00 || MARTIN | 1250.00 || BLAKE | 2850.00 || CLARK | 2450.00 || SCOTT | 3000.00 || KING | 5000.00 || TURNER | 1500.00 || ADAMS | 1100.00 || JAMES | 950.00 || FORD | 3000.00 || MILLER | 1300.00 |+--------+---------+mysql> select * from salgrade; s+-------+-------+-------+ | GRADE | LOSAL | HISAL |+-------+-------+-------+| 1 | 700 | 1200 || 2 | 1201 | 1400 || 3 | 1401 | 2000 || 4 | 2001 | 3000 || 5 | 3001 | 9999 |+-------+-------+-------+select e.ename,e.sal,s.gradefromemp ejoinsalgrade sone.sal between s.losal and s.hisal;// inner可以省略select e.ename,e.sal,s.gradefromemp einner joinsalgrade sone.sal between s.losal and s.hisal;+--------+---------+-------+| ename | sal | grade |+--------+---------+-------+| SMITH | 800.00 | 1 || ALLEN | 1600.00 | 3 || WARD | 1250.00 | 2 || JONES | 2975.00 | 4 || MARTIN | 1250.00 | 2 || BLAKE | 2850.00 | 4 || CLARK | 2450.00 | 4 || SCOTT | 3000.00 | 4 || KING | 5000.00 | 5 || TURNER | 1500.00 | 3 || ADAMS | 1100.00 | 1 || JAMES | 950.00 | 1 || FORD | 3000.00 | 4 || MILLER | 1300.00 | 2 |+--------+---------+-------+最大的特点是：一张表看做两张表。自己连接自己。案例：找出每个员工的上级领导，要求显示员工名和对应的领导名。mysql> select empno,ename,mgr from emp;emp a 员工表+-------+--------+------+| empno | ename | mgr |+-------+--------+------+| 7369 | SMITH | 7902 || 7499 | ALLEN | 7698 || 7521 | WARD | 7698 || 7566 | JONES | 7839 || 7654 | MARTIN | 7698 || 7698 | BLAKE | 7839 || 7782 | CLARK | 7839 || 7788 | SCOTT | 7566 || 7839 | KING | NULL || 7844 | TURNER | 7698 || 7876 | ADAMS | 7788 || 7900 | JAMES | 7698 || 7902 | FORD | 7566 || 7934 | MILLER | 7782 |+-------+--------+------+emp b 领导表+-------+--------+| empno | ename |+-------+--------+| 7566 | JONES || 7698 | BLAKE || 7782 | CLARK || 7788 | SCOTT || 7839 | KING || 7902 | FORD |+-------+--------+员工的领导编号 = 领导的员工编号select a.ename as '员工名',b.ename as '领导名'fromemp ainner joinemp bona.mgr = b.empno;+--------+--------+| 员工名 | 领导名 |+--------+--------+| SMITH | FORD || ALLEN | BLAKE || WARD | BLAKE || JONES | KING || MARTIN | BLAKE || BLAKE | KING || CLARK | KING || SCOTT | JONES || TURNER | BLAKE || ADAMS | SCOTT || JAMES | BLAKE || FORD | JONES || MILLER | CLARK |+--------+--------+什么是外连接，和内连接有什么区别？内连接：假设A和B表进行连接，使用内连接的话，凡是A表和B表能够匹配上的记录查询出来，这就是内连接。AB两张表没有主副之分，两张表是平等的。外连接：假设A和B表进行连接，使用外连接的话，AB两张表中有一张表是主表，一张表是副表，主要查询主表中的数据，捎带着查询副表，当副表中的数据没有和主表中的数据匹配上，副表自动模拟出NULL与之匹配。外连接的分类？左外连接（左连接）：表示左边的这张表是主表。右外连接（右连接）：表示右边的这张表是主表。左连接有右连接的写法，右连接也会有对应的左连接的写法。案例：找出每个员工的上级领导？（所有员工必须全部查询出来。）emp a 员工表+-------+--------+------+| empno | ename | mgr |+-------+--------+------+| 7369 | SMITH | 7902 || 7499 | ALLEN | 7698 || 7521 | WARD | 7698 || 7566 | JONES | 7839 || 7654 | MARTIN | 7698 || 7698 | BLAKE | 7839 || 7782 | CLARK | 7839 || 7788 | SCOTT | 7566 || 7839 | KING | NULL || 7844 | TURNER | 7698 || 7876 | ADAMS | 7788 || 7900 | JAMES | 7698 || 7902 | FORD | 7566 || 7934 | MILLER | 7782 |+-------+--------+------+emp b 领导表+-------+--------+| empno | ename |+-------+--------+| 7566 | JONES || 7698 | BLAKE || 7782 | CLARK || 7788 | SCOTT || 7839 | KING || 7902 | FORD |+-------+--------+内连接：select a.ename '员工', b.ename '领导'fromemp ajoinemp bona.mgr = b.empno;外连接：（左外连接/左连接）select a.ename '员工', b.ename '领导'fromemp aleft joinemp bona.mgr = b.empno;// outer是可以省略的。select a.ename '员工', b.ename '领导'fromemp aleft outer joinemp bona.mgr = b.empno;外连接：（右外连接/右连接）select a.ename '员工', b.ename '领导'fromemp bright joinemp aona.mgr = b.empno;// outer可以省略。select a.ename '员工', b.ename '领导'fromemp bright outer joinemp aona.mgr = b.empno;+--------+-------+| 员工 | 领导 |+--------+-------+| SMITH | FORD || ALLEN | BLAKE || WARD | BLAKE || JONES | KING || MARTIN | BLAKE || BLAKE | KING || CLARK | KING || SCOTT | JONES || KING | NULL || TURNER | BLAKE || ADAMS | SCOTT || JAMES | BLAKE || FORD | JONES || MILLER | CLARK |+--------+-------+外连接最重要的特点是：主表的数据无条件的全部查询出来。案例：找出哪个部门没有员工？EMP表+-------+--------+-----------+------+------------+---------+---------+--------+| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |+-------+--------+-----------+------+------------+---------+---------+--------+| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 || 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 || 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 || 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 || 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 || 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 || 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 || 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 || 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 || 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 || 7876 | ADAMS | CLERK | 7788 | 1987-05-23 | 1100.00 | NULL | 20 || 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 || 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 || 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |+-------+--------+-----------+------+------------+---------+---------+--------+DEPT+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+select d.*fromemp eright joindept done.deptno = d.deptnowheree.empno is null;+--------+------------+--------+| DEPTNO | DNAME | LOC |+--------+------------+--------+| 40 | OPERATIONS | BOSTON |+--------+------------+--------+案例：找出每一个员工的部门名称以及工资等级。EMP e+-------+--------+---------+--------+| empno | ename | sal | deptno |+-------+--------+---------+--------+| 7369 | SMITH | 800.00 | 20 || 7499 | ALLEN | 1600.00 | 30 || 7521 | WARD | 1250.00 | 30 || 7566 | JONES | 2975.00 | 20 || 7654 | MARTIN | 1250.00 | 30 || 7698 | BLAKE | 2850.00 | 30 || 7782 | CLARK | 2450.00 | 10 || 7788 | SCOTT | 3000.00 | 20 || 7839 | KING | 5000.00 | 10 || 7844 | TURNER | 1500.00 | 30 || 7876 | ADAMS | 1100.00 | 20 || 7900 | JAMES | 950.00 | 30 || 7902 | FORD | 3000.00 | 20 || 7934 | MILLER | 1300.00 | 10 |+-------+--------+---------+--------+DEPT d+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+SALGRADE s+-------+-------+-------+| GRADE | LOSAL | HISAL |+-------+-------+-------+| 1 | 700 | 1200 || 2 | 1201 | 1400 || 3 | 1401 | 2000 || 4 | 2001 | 3000 || 5 | 3001 | 9999 |+-------+-------+-------+注意，解释一下：....AjoinBjoinCon...表示：A表和B表先进行表连接，连接之后A表继续和C表进行连接。select e.ename,d.dname,s.gradefromemp ejoindept done.deptno = d.deptnojoinsalgrade sone.sal between s.losal and s.hisal;+--------+------------+-------+| ename | dname | grade |+--------+------------+-------+| SMITH | RESEARCH | 1 || ALLEN | SALES | 3 || WARD | SALES | 2 || JONES | RESEARCH | 4 || MARTIN | SALES | 2 || BLAKE | SALES | 4 || CLARK | ACCOUNTING | 4 || SCOTT | RESEARCH | 4 || KING | ACCOUNTING | 5 || TURNER | SALES | 3 || ADAMS | RESEARCH | 1 || JAMES | SALES | 1 || FORD | RESEARCH | 4 || MILLER | ACCOUNTING | 2 |+--------+------------+-------+案例：找出每一个员工的部门名称、工资等级、以及上级领导。select e.ename '员工',d.dname,s.grade,e1.ename '领导'fromemp ejoindept done.deptno = d.deptnojoinsalgrade sone.sal between s.losal and s.hisalleft joinemp e1one.mgr = e1.empno;+--------+------------+-------+-------+| 员工 | dname | grade | 领导 |+--------+------------+-------+-------+| SMITH | RESEARCH | 1 | FORD || ALLEN | SALES | 3 | BLAKE || WARD | SALES | 2 | BLAKE || JONES | RESEARCH | 4 | KING || MARTIN | SALES | 2 | BLAKE || BLAKE | SALES | 4 | KING || CLARK | ACCOUNTING | 4 | KING || SCOTT | RESEARCH | 4 | JONES || KING | ACCOUNTING | 5 | NULL || TURNER | SALES | 3 | BLAKE || ADAMS | RESEARCH | 1 | SCOTT || JAMES | SALES | 1 | BLAKE || FORD | RESEARCH | 4 | JONES || MILLER | ACCOUNTING | 2 | CLARK |+--------+------------+-------+-------+select语句当中嵌套select语句，被嵌套的select语句是子查询。子查询可以出现在哪里？select..(select).from..(select).where..(select).案例：找出高于平均薪资的员工信息。select * from emp where sal > avg(sal); //错误的写法，where后面不能直接使用分组函数。第一步：找出平均薪资select avg(sal) from emp;+-------------+| avg(sal) |+-------------+| 2073.214286 |+-------------+第二步：where过滤select * from emp where sal > 2073.214286;+-------+-------+-----------+------+------------+---------+------+--------+| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |+-------+-------+-----------+------+------------+---------+------+--------+| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975.00 | NULL | 20 || 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850.00 | NULL | 30 || 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450.00 | NULL | 10 || 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000.00 | NULL | 20 || 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 || 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |+-------+-------+-----------+------+------------+---------+------+--------+第一步和第二步合并：select * from emp where sal > (select avg(sal) from emp);案例：找出每个部门平均薪水的等级。第一步：找出每个部门平均薪水（按照部门编号分组，求sal的平均值）select deptno,avg(sal) as avgsal from emp group by deptno;+--------+-------------+| deptno | avgsal |+--------+-------------+| 10 | 2916.666667 || 20 | 2175.000000 || 30 | 1566.666667 |+--------+-------------+第二步：将以上的查询结果当做临时表t，让t表和salgrade s表连接，条件是：t.avgsal between s.losal and s.hisalselect t.*,s.gradefrom(select deptno,avg(sal) as avgsal from emp group by deptno) tjoinsalgrade sont.avgsal between s.losal and s.hisal;+--------+-------------+-------+| deptno | avgsal | grade |+--------+-------------+-------+| 30 | 1566.666667 | 3 || 10 | 2916.666667 | 4 || 20 | 2175.000000 | 4 |+--------+-------------+-------+案例：找出每个部门平均的薪水等级。第一步：找出每个员工的薪水等级。select e.ename,e.sal,e.deptno,s.grade from emp e join salgrade s on e.sal between s.losal and s.hisal;+--------+---------+--------+-------+| ename | sal | deptno | grade |+--------+---------+--------+-------+| SMITH | 800.00 | 20 | 1 || ALLEN | 1600.00 | 30 | 3 || WARD | 1250.00 | 30 | 2 || JONES | 2975.00 | 20 | 4 || MARTIN | 1250.00 | 30 | 2 || BLAKE | 2850.00 | 30 | 4 || CLARK | 2450.00 | 10 | 4 || SCOTT | 3000.00 | 20 | 4 || KING | 5000.00 | 10 | 5 || TURNER | 1500.00 | 30 | 3 || ADAMS | 1100.00 | 20 | 1 || JAMES | 950.00 | 30 | 1 || FORD | 3000.00 | 20 | 4 || MILLER | 1300.00 | 10 | 2 |+--------+---------+--------+-------+第二步：基于以上结果，继续按照deptno分组，求grade平均值。select e.deptno,avg(s.grade)from emp e join salgrade s on e.sal between s.losal and s.hisalgroup bye.deptno;+--------+--------------+| deptno | avg(s.grade) |+--------+--------------+| 10 | 3.6667 || 20 | 2.8000 || 30 | 2.5000 |+--------+--------------+案例：找出每个员工所在的部门名称，要求显示员工名和部门名。select e.ename,d.dnamefromemp ejoindept done.deptno = d.deptno;select e.ename,(select d.dname from dept d where e.deptno = d.deptno) as dname from emp e;+--------+------------+| ename | dname |+--------+------------+| SMITH | RESEARCH || ALLEN | SALES || WARD | SALES || JONES | RESEARCH || MARTIN | SALES || BLAKE | SALES || CLARK | ACCOUNTING || SCOTT | RESEARCH || KING | ACCOUNTING || TURNER | SALES || ADAMS | RESEARCH || JAMES | SALES || FORD | RESEARCH || MILLER | ACCOUNTING |+--------+------------+可以将查询结果集相加案例：找出工作岗位是SALESMAN和MANAGER的员工？第一种：select ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';第二种：select ename,job from emp where job in('MANAGER','SALESMAN');+--------+----------+| ename | job |+--------+----------+| ALLEN | SALESMAN || WARD | SALESMAN || JONES | MANAGER || MARTIN | SALESMAN || BLAKE | MANAGER || CLARK | MANAGER || TURNER | SALESMAN |+--------+----------+第三种：unionselect ename,job from emp where job = 'MANAGER'unionselect ename,job from emp where job = 'SALESMAN';+--------+----------+| ename | job |+--------+----------+| JONES | MANAGER || BLAKE | MANAGER || CLARK | MANAGER || ALLEN | SALESMAN || WARD | SALESMAN || MARTIN | SALESMAN || TURNER | SALESMAN |+--------+----------+两张不相干的表中的数据拼接在一起显示？select ename from empunionselect dname from dept;+------------+| ename |+------------+| SMITH || ALLEN || WARD || JONES || MARTIN || BLAKE || CLARK || SCOTT || KING || TURNER || ADAMS || JAMES || FORD || MILLER || ACCOUNTING || RESEARCH || SALES || OPERATIONS |+------------+mysql> select ename,sal from emp -> union -> select dname from dept;ERROR 1222 (21000): The used SELECT statements have a different number of columnslimit是mysql特有的，其他数据库中没有，不通用。（Oracle中有一个相同的机制，叫做rownum）limit取结果集中的部分数据，这时它的作用。语法机制：limit startIndex, lengthstartIndex表示起始位置，从0开始，0表示第一条数据。length表示取几个案例：取出工资前5名的员工（思路：降序取前5个）select ename,sal from emp order by sal desc;取前5个：select ename,sal from emp order by sal desc limit 0, 5;select ename,sal from emp order by sal desc limit 5;limit是sql语句最后执行的一个环节：select5...from1...where2...group by3...having4...order by6...limit7...;案例：找出工资排名在第4到第9名的员工？select ename,sal from emp order by sal desc limit 3,6;+--------+---------+| ename | sal |+--------+---------+| JONES | 2975.00 || BLAKE | 2850.00 || CLARK | 2450.00 || ALLEN | 1600.00 || TURNER | 1500.00 || MILLER | 1300.00 |+--------+---------+通用的标准分页sql？每页显示3条记录：第1页：0, 3第2页：3, 3第3页：6, 3第4页：9, 3第5页：12, 3每页显示pageSize条记录：第pageNo页：(pageNo - 1) * pageSize, pageSizepageSize是什么？是每页显示多少条记录pageNo是什么？显示第几页java代码{int pageNo = 2; // 页码是2int pageSize = 10; // 每页显示10条limit (pageNo - 1) * pageSize, pageSize}建表语句的语法格式：create table 表名(字段名1 数据类型,字段名2 数据类型,字段名3 数据类型,....);关于MySQL当中字段的数据类型？以下只说常见的int整数型(java中的int)bigint长整型(java中的long)float浮点型(java中的float double)char定长字符串(String)varchar可变长字符串(StringBuffer/StringBuilder)date日期类型 （对应Java中的java.sql.Date类型）BLOB二进制大对象（存储图片、视频等流媒体信息） Binary Large OBject （对应java中的Object）CLOB字符大对象（存储较大文本，比如，可以存储4G的字符串。） Character Large OBject（对应java中的Object）......char和varchar怎么选择？在实际的开发中，当某个字段中的数据长度不发生改变的时候，是定长的，例如：性别、生日等都是采用char。当一个字段的数据长度不确定，例如：简介、姓名等都是采用varchar。BLOB和CLOB类型的使用？电影表: t_movieid(int)name(varchar)playtime(date/char)haibao(BLOB)history(CLOB)----------------------------------------------------------------------------------------1蜘蛛侠23表名在数据库当中一般建议以：t_或者tbl_开始。创建学生表：学生信息包括：学号、姓名、性别、班级编号、生日学号：bigint姓名：varchar性别：char班级编号：int生日：charcreate table t_student(no bigint,name varchar(255),sex char(1),classno varchar(255),birth char(10));语法格式：insert into 表名(字段名1,字段名2,字段名3,....) values(值1,值2,值3,....)要求：字段的数量和值的数量相同，并且数据类型要对应相同。insert into t_student(no,name,sex,classno,birth) values(1,'zhangsan','1','gaosan1ban');ERROR 1136 (21S01): Column count doesn't match value count at row 1insert into t_student(no,name,sex,classno,birth) values(1,'zhangsan','1','gaosan1ban', '1950-10-12');mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 |+------+----------+------+------------+------------+insert into t_student(name,sex,classno,birth,no) values('lisi','1','gaosan1ban', '1950-10-12',2);mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 || 2 | lisi | 1 | gaosan1ban | 1950-10-12 |+------+----------+------+------------+------------+insert into t_student(name) values('wangwu'); // 除name字段之外，剩下的所有字段自动插入NULL。mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 || 2 | lisi | 1 | gaosan1ban | 1950-10-12 || NULL | wangwu | NULL | NULL | NULL |+------+----------+------+------------+------------+insert into t_student(no) values(3); mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| 1 | zhangsan | 1 | gaosan1ban | 1950-10-12 || 2 | lisi | 1 | gaosan1ban | 1950-10-12 || NULL | wangwu | NULL | NULL | NULL || 3 | NULL | NULL | NULL | NULL |+------+----------+------+------------+------------+drop table if exists t_student; // 当这个表存在的话删除。create table t_student(no bigint,name varchar(255),sex char(1) default 1,classno varchar(255),birth char(10));insert into t_student(name) values('zhangsan');mysql> select * from t_student;+------+----------+------+---------+-------+| no | name | sex | classno | birth |+------+----------+------+---------+-------+| NULL | zhangsan | 1 | NULL | NULL |+------+----------+------+---------+-------+需要注意的地方：当一条insert语句执行成功之后，表格当中必然会多一行记录。即使多的这一行记录当中某些字段是NULL，后期也没有办法在执行insert语句插入数据了，只能使用update进行更新。// 字段可以省略不写，但是后面的value对数量和顺序都有要求。insert into t_student values(1,'jack','0','gaosan2ban','1986-10-23');mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| NULL | zhangsan | 1 | NULL | NULL || 1 | jack | 0 | gaosan2ban | 1986-10-23 |+------+----------+------+------------+------------+insert into t_student values(1,'jack','0','gaosan2ban');ERROR 1136 (21S01): Column count doesn't match value count at row 1// 一次插入多行数据insert into t_student(no,name,sex,classno,birth) values(3,'rose','1','gaosi2ban','1952-12-14'),(4,'laotie','1','gaosi2ban','1955-12-14');mysql> select * from t_student;+------+----------+------+------------+------------+| no | name | sex | classno | birth |+------+----------+------+------------+------------+| NULL | zhangsan | 1 | NULL | NULL || 1 | jack | 0 | gaosan2ban | 1986-10-23 || 3 | rose | 1 | gaosi2ban | 1952-12-14 || 4 | laotie | 1 | gaosi2ban | 1955-12-14 |+------+----------+------+------------+------------+语法：create table 表名 as select语句;将查询结果当做表创建出来。mysql> insert into dept1 select * from dept;mysql> select * from dept1;+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON || 10 | ACCOUNTING | NEW YORK || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+语法格式：update 表名 set 字段名1=值1,字段名2=值2... where 条件;注意：没有条件整张表数据全部更新。案例：将部门10的LOC修改为SHANGHAI，将部门名称修改为RENSHIBUupdate dept1 set loc = 'SHANGHAI', dname = 'RENSHIBU' where deptno = 10;mysql> select * from dept1;+--------+------------+----------+| DEPTNO | DNAME | LOC |+--------+------------+----------+| 10 | RENSHIBU | SHANGHAI || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON || 10 | RENSHIBU | SHANGHAI || 20 | RESEARCH | DALLAS || 30 | SALES | CHICAGO || 40 | OPERATIONS | BOSTON |+--------+------------+----------+更新所有记录update dept1 set loc = 'x', dname = 'y';mysql> select * from dept1;+--------+-------+------+| DEPTNO | DNAME | LOC |+--------+-------+------+| 10 | y | x || 20 | y | x || 30 | y | x || 40 | y | x || 10 | y | x || 20 | y | x || 30 | y | x || 40 | y | x |+--------+-------+------+语法格式：delete from 表名 where 条件;注意：没有条件全部删除。删除10部门数据？delete from dept1 where deptno = 10;删除所有记录？delete from dept1;怎么删除大表中的数据？（重点）truncate table 表名; // 表被截断，不可回滚。永久丢失。删除表？drop table 表名; // 这个通用。drop table if exists 表名; // oracle不支持这种写法。这里不讲了，大家使用工具完成即可，因为在实际开发中表一旦设计好之后，对表结构的修改是很少的，修改表结构就是对之前的设计进行了否定，即使需要修改表结构，我们也可以直接使用工具操作。修改表结构的语句不会出现在Java代码当中。出现在java代码当中的sql包括：insert delete update select（这些都是表中的数据操作。）增删改查有一个术语：CRUD操作Create（增） Retrieve（检索） Update（修改） Delete（删除）在创建表的时候，可以给表的字段添加相应的约束，添加约束的目的是为了保证表中数据的合法性、有效性、完整性。常见的约束有哪些呢？非空约束(not null)：约束的字段不能为NULL唯一约束(unique)：约束的字段不能重复主键约束(primary key)：约束的字段既不能为NULL，也不能重复（简称PK）外键约束(foreign key)：...（简称FK）检查约束(check)：注意Oracle数据库有check约束，但是mysql没有，目前mysql不支持该约束。drop table if exists t_user;create table t_user(id int,username varchar(255) not null,password varchar(255));insert into t_user(id,password) values(1,'123');ERROR 1364 (HY000): Field 'username' doesn't have a default valueinsert into t_user(id,username,password) values(1,'lisi','123');* 唯一约束修饰的字段具有唯一性，不能重复。但可以为NULL。* 案例：给某一列添加uniquedrop table if exists t_user;create table t_user(id int,username varchar(255) unique // 列级约束);insert into t_user values(1,'zhangsan');insert into t_user values(2,'zhangsan');ERROR 1062 (23000): Duplicate entry 'zhangsan' for key 'username'insert into t_user(id) values(2);insert into t_user(id) values(3);insert into t_user(id) values(4);* 案例：给两个列或者多个列添加uniquedrop table if exists t_user;create table t_user(id int, usercode varchar(255),username varchar(255),unique(usercode,username) // 多个字段联合起来添加1个约束unique 【表级约束】);insert into t_user values(1,'111','zs');insert into t_user values(2,'111','ls');insert into t_user values(3,'222','zs');select * from t_user;insert into t_user values(4,'111','zs');ERROR 1062 (23000): Duplicate entry '111-zs' for key 'usercode'drop table if exists t_user;create table t_user(id int, usercode varchar(255) unique,username varchar(255) unique);insert into t_user values(1,'111','zs');insert into t_user values(2,'111','ls');ERROR 1062 (23000): Duplicate entry '111' for key 'usercode'* 注意：not null约束只有列级约束。没有表级约束。* 怎么给一张表添加主键约束呢？drop table if exists t_user;create table t_user(id int primary key, // 列级约束username varchar(255),email varchar(255));insert into t_user(id,username,email) values(1,'zs','zs@123.com');insert into t_user(id,username,email) values(2,'ls','ls@123.com');insert into t_user(id,username,email) values(3,'ww','ww@123.com');select * from t_user;+----+----------+------------+| id | username | email |+----+----------+------------+| 1 | zs | zs@123.com || 2 | ls | ls@123.com || 3 | ww | ww@123.com |+----+----------+------------+insert into t_user(id,username,email) values(1,'jack','jack@123.com');ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'insert into t_user(username,email) values('jack','jack@123.com');ERROR 1364 (HY000): Field 'id' doesn't have a default value根据以上的测试得出：id是主键，因为添加了主键约束，主键字段中的数据不能为NULL，也不能重复。主键的特点：不能为NULL，也不能重复。* 主键相关的术语？主键约束 : primary key主键字段 : id字段添加primary key之后，id叫做主键字段主键值 : id字段中的每一个值都是主键值。* 主键有什么作用？- 表的设计三范式中有要求，第一范式就要求任何一张表都应该有主键。- 主键的作用：主键值是这行记录在这张表当中的唯一标识。（就像一个人的身份证号码一样。）* 主键的分类？根据主键字段的字段数量来划分：单一主键（推荐的，常用的。）复合主键(多个字段联合起来添加一个主键约束)（复合主键不建议使用，因为复合主键违背三范式。）根据主键性质来划分：自然主键：主键值最好就是一个和业务没有任何关系的自然数。（这种方式是推荐的）业务主键：主键值和系统的业务挂钩，例如：拿着银行卡的卡号做主键，拿着身份证号码作为主键。（不推荐用） 最好不要拿着和业务挂钩的字段作为主键。因为以后的业务一旦发生改变的时候，主键值可能也需要 随着发生变化，但有的时候没有办法变化，因为变化可能会导致主键值重复。* 一张表的主键约束只能有1个。（必须记住）* 使用表级约束方式定义主键：drop table if exists t_user;create table t_user(id int,username varchar(255),primary key(id));insert into t_user(id,username) values(1,'zs');insert into t_user(id,username) values(2,'ls');insert into t_user(id,username) values(3,'ws');insert into t_user(id,username) values(4,'cs');select * from t_user;insert into t_user(id,username) values(4,'cx');ERROR 1062 (23000): Duplicate entry '4' for key 'PRIMARY'以下内容是演示以下复合主键，不需要掌握：drop table if exists t_user;create table t_user(id int,username varchar(255),password varchar(255),primary key(id,username));insert .......* mysql提供主键值自增：（非常重要。）drop table if exists t_user;create table t_user(id int primary key auto_increment, // id字段自动维护一个自增的数字，从1开始，以1递增。username varchar(255));insert into t_user(username) values('a');insert into t_user(username) values('b');insert into t_user(username) values('c');insert into t_user(username) values('d');insert into t_user(username) values('e');insert into t_user(username) values('f');select * from t_user;提示:Oracle当中也提供了一个自增机制，叫做：序列（sequence）对象。* 关于外键约束的相关术语：外键约束: foreign key外键字段：添加有外键约束的字段外键值：外键字段中的每一个值。* 业务背景：请设计数据库表，用来维护学生和班级的信息？第一种方案：一张表存储所有数据no(pk)nameclassnoclassname-------------------------------------------------------------------------------------------1zs1101北京大兴区经济技术开发区亦庄二中高三1班2zs2101北京大兴区经济技术开发区亦庄二中高三1班3zs3102北京大兴区经济技术开发区亦庄二中高三2班4zs4102北京大兴区经济技术开发区亦庄二中高三2班5zs5102北京大兴区经济技术开发区亦庄二中高三2班缺点：冗余。【不推荐】第二种方案：两张表（班级表和学生表）t_class 班级表cno(pk)cname--------------------------------------------------------101北京大兴区经济技术开发区亦庄二中高三1班102北京大兴区经济技术开发区亦庄二中高三2班t_student 学生表sno(pk)snameclassno(该字段添加外键约束fk)------------------------------------------------------------1zs11012zs21013zs31024zs41025zs5102* 将以上表的建表语句写出来：t_student中的classno字段引用t_class表中的cno字段，此时t_student表叫做子表。t_class表叫做父表。顺序要求：删除数据的时候，先删除子表，再删除父表。添加数据的时候，先添加父表，在添加子表。创建表的时候，先创建父表，再创建子表。删除表的时候，先删除子表，在删除父表。drop table if exists t_student;drop table if exists t_class;create table t_class(cno int,cname varchar(255),primary key(cno));create table t_student(sno int,sname varchar(255),classno int,primary key(sno),foreign key(classno) references t_class(cno));insert into t_class values(101,'xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx');insert into t_class values(102,'yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy');insert into t_student values(1,'zs1',101);insert into t_student values(2,'zs2',101);insert into t_student values(3,'zs3',102);insert into t_student values(4,'zs4',102);insert into t_student values(5,'zs5',102);insert into t_student values(6,'zs6',102);select * from t_class;select * from t_student;insert into t_student values(7,'lisi',103);ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`bjpowernode`.INT `t_student_ibfk_1` FOREIGN KEY (`classno`) REFERENCES `t_class` (`cno`))* 外键值可以为NULL？外键可以为NULL。* 外键字段引用其他表的某个字段的时候，被引用的字段必须是主键吗？注意：被引用的字段不一定是主键，但至少具有unique约束。CREATE TABLE `t_x` ( `id` int(11) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8;注意：在MySQL当中，凡是标识符是可以使用飘号括起来的。最好别用，不通用。建表的时候可以指定存储引擎，也可以指定字符集。mysql默认使用的存储引擎是InnoDB方式。默认采用的字符集是UTF8存储引擎这个名字只有在mysql中存在。（Oracle中有对应的机制，但是不叫做存储引擎。Oracle中没有特殊的名字，就是“表的存储方式”）mysql支持很多存储引擎，每一个存储引擎都对应了一种不同的存储方式。每一个存储引擎都有自己的优缺点，需要在合适的时机选择合适的存储引擎。show engines \Gmysql 5.5.36版本支持的存储引擎有9个：*************************** 1. row ***************************Engine: FEDERATED Support: NO Comment: Federated MySQL storage engineTransactions: NULL XA: NULL Savepoints: NULL*************************** 2. row ***************************Engine: MRG_MYISAM Support: YES Comment: Collection of identical MyISAM tablesTransactions: NO XA: NO Savepoints: NO*************************** 3. row ***************************Engine: MyISAM Support: YES Comment: MyISAM storage engineTransactions: NO XA: NO Savepoints: NO*************************** 4. row ***************************Engine: BLACKHOLE Support: YES Comment: /dev/null storage engine (anything you write to it disappears)Transactions: NO XA: NO Savepoints: NO*************************** 5. row ***************************Engine: CSV Support: YES Comment: CSV storage engineTransactions: NO XA: NO Savepoints: NO*************************** 6. row ***************************Engine: MEMORY Support: YES Comment: Hash based, stored in memory, useful for temporary tablesTransactions: NO XA: NO Savepoints: NO*************************** 7. row ***************************Engine: ARCHIVE Support: YES Comment: Archive storage engineTransactions: NO XA: NO Savepoints: NO*************************** 8. row ***************************Engine: InnoDB Support: DEFAULT Comment: Supports transactions, row-level locking, and foreign keysTransactions: YES XA: YES Savepoints: YES*************************** 9. row ***************************Engine: PERFORMANCE_SCHEMA Support: YES Comment: Performance SchemaTransactions: NO XA: NO Savepoints: NO Engine: MyISAM Support: YES Comment: MyISAM storage engineTransactions: NO XA: NO Savepoints: NOMyISAM这种存储引擎不支持事务。MyISAM是mysql最常用的存储引擎，但是这种引擎不是默认的。MyISAM采用三个文件组织一张表：xxx.frm（存储格式的文件）xxx.MYD（存储表中数据的文件）xxx.MYI（存储表中索引的文件）优点：可被压缩，节省存储空间。并且可以转换为只读表，提高检索效率。缺点：不支持事务。----------------------------------------------------------------------------- Engine: InnoDB Support: DEFAULT Comment: Supports transactions, row-level locking, and foreign keysTransactions: YES XA: YES Savepoints: YES优点：支持事务、行级锁、外键等。这种存储引擎数据的安全得到保障。表的结构存储在xxx.frm文件中数据存储在tablespace这样的表空间中（逻辑概念），无法被压缩，无法转换成只读。这种InnoDB存储引擎在MySQL数据库崩溃之后提供自动恢复机制。InnoDB支持级联删除和级联更新。------------------------------------------------------------------------------------- Engine: MEMORY Support: YES Comment: Hash based, stored in memory, useful for temporary tablesTransactions: NO XA: NO Savepoints: NO缺点：不支持事务。数据容易丢失。因为所有数据和索引都是存储在内存当中的。优点：查询速度最快。以前叫做HEPA引擎。一个事务是一个完整的业务逻辑单元，不可再分。比如：银行账户转账，从A账户向B账户转账10000.需要执行两条update语句：update t_act set balance = balance - 10000 where actno = 'act-001';update t_act set balance = balance + 10000 where actno = 'act-002';以上两条DML语句必须同时成功，或者同时失败，不允许出现一条成功，一条失败。要想保证以上的两条DML语句同时成功或者同时失败，那么就需要使用数据库的“事务机制”。为什么？因为它们这三个语句都是和数据库表当中的“数据”相关的。事务的存在是为了保证数据的完整性，安全性。假设所有的业务都能使用1条DML语句搞定，还需要事务机制吗？不需要事务。但实际情况不是这样的，通常一个“事儿（事务【业务】）”需要多条DML语句共同联合完成。事务包括四大特性：ACIDA: 原子性：事务是最小的工作单元，不可再分。C: 一致性：事务必须保证多条DML语句同时成功或者同时失败。I：隔离性：事务A与事务B之间具有隔离。D：持久性：持久性说的是最终数据必须持久化到硬盘文件中，事务才算成功的结束。事务隔离性存在隔离级别，理论上隔离级别包括4个：第一级别：读未提交（read uncommitted）对方事务还没有提交，我们当前事务可以读取到对方未提交的数据。读未提交存在脏读（Dirty Read）现象：表示读到了脏的数据。第二级别：读已提交（read committed）对方事务提交之后的数据我方可以读取到。这种隔离级别解决了: 脏读现象没有了。读已提交存在的问题是：不可重复读。第三级别：可重复读（repeatable read）这种隔离级别解决了：不可重复读问题。这种隔离级别存在的问题是：读取到的数据是幻象。第四级别：序列化读/串行化读（serializable） 解决了所有问题。效率低。需要事务排队。oracle数据库默认的隔离级别是：读已提交。mysql数据库默认的隔离级别是：可重复读。* mysql事务默认情况下是自动提交的。（什么是自动提交？只要执行任意一条DML语句则提交一次。）怎么关闭自动提交？start transaction;* 准备表：drop table if exists t_user;create table t_user(id int primary key auto_increment,username varchar(255));* 演示：mysql中的事务是支持自动提交的，只要执行一条DML，则提交一次。mysql> insert into t_user(username) values('zs');Query OK, 1 row affected (0.03 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs |+----+----------+1 row in set (0.00 sec)mysql> rollback;Query OK, 0 rows affected (0.00 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs |+----+----------+1 row in set (0.00 sec)* 演示：使用start transaction;关闭自动提交机制。mysql> start transaction;Query OK, 0 rows affected (0.00 sec)mysql> insert into t_user(username) values('lisi');Query OK, 1 row affected (0.00 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs || 2 | lisi |+----+----------+2 rows in set (0.00 sec)mysql> insert into t_user(username) values('wangwu');Query OK, 1 row affected (0.00 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs || 2 | lisi || 3 | wangwu |+----+----------+3 rows in set (0.00 sec)mysql> rollback;Query OK, 0 rows affected (0.02 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs |+----+----------+1 row in set (0.00 sec)--------------------------------------------------------------------mysql> start transaction;Query OK, 0 rows affected (0.00 sec)mysql> insert into t_user(username) values('wangwu');Query OK, 1 row affected (0.00 sec)mysql> insert into t_user(username) values('rose');Query OK, 1 row affected (0.00 sec)mysql> insert into t_user(username) values('jack');Query OK, 1 row affected (0.00 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs || 4 | wangwu || 5 | rose || 6 | jack |+----+----------+4 rows in set (0.00 sec)mysql> commit;Query OK, 0 rows affected (0.04 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs || 4 | wangwu || 5 | rose || 6 | jack |+----+----------+4 rows in set (0.00 sec)mysql> rollback;Query OK, 0 rows affected (0.00 sec)mysql> select * from t_user;+----+----------+| id | username |+----+----------+| 1 | zs || 4 | wangwu || 5 | rose || 6 | jack |+----+----------+4 rows in set (0.00 sec)* 演示两个事务，假如隔离级别演示第1级别：读未提交set global transaction isolation level read uncommitted;演示第2级别：读已提交set global transaction isolation level read committed;演示第3级别：可重复读set global transaction isolation level repeatable read;* mysql远程登录：mysql -h192.168.151.18 -uroot -p444索引就相当于一本书的目录，通过目录可以快速的找到对应的资源。在数据库方面，查询一张表的时候有两种检索方式：第一种方式：全表扫描第二种方式：根据索引检索（效率很高）索引为什么可以提高检索效率呢？其实最根本的原理是缩小了扫描的范围。索引虽然可以提高检索效率，但是不能随意的添加索引，因为索引也是数据库当中的对象，也需要数据库不断的维护。是有维护成本的。比如，表中的数据经常被修改这样就不适合添加索引，因为数据一旦修改，索引需要重新排序，进行维护。添加索引是给某一个字段，或者说某些字段添加索引。select ename,sal from emp where ename = 'SMITH';当ename字段上没有添加索引的时候，以上sql语句会进行全表扫描，扫描ename字段中所有的值。当ename字段上添加索引的时候，以上sql语句会根据索引扫描，快速定位。创建索引对象：create index 索引名称 on 表名(字段名);删除索引对象：drop index 索引名称 on 表名;* 数据量庞大。（根据客户的需求，根据线上的环境）* 该字段很少的DML操作。（因为字段进行修改操作，索引也需要维护）* 该字段经常出现在where子句中。（经常根据哪个字段查询）注意：主键和具有unique约束的字段自动会添加索引。根据主键查询效率较高。尽量根据主键检索。mysql> explain select ename,sal from emp where sal = 5000;+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+| 1 | SIMPLE | emp | ALL | NULL | NULL | NULL | NULL | 14 | Using where |+----+-------------+-------+------+---------------+------+---------+------+------+-------------+给薪资sal字段添加索引：create index emp_sal_index on emp(sal);mysql> explain select ename,sal from emp where sal = 5000;+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+| 1 | SIMPLE | emp | ref | emp_sal_index | emp_sal_index | 9 | const | 1 | Using where |+----+-------------+-------+------+---------------+---------------+---------+-------+------+-------------+B + Tree通过B Tree缩小扫描范围，底层索引进行了排序，分区，索引会携带数据在表中的“物理地址”，最终通过索引检索到数据之后，获取到关联的物理地址，通过物理地址定位表中的数据，效率是最高的。select ename from emp where ename = 'SMITH';通过索引转换为：select ename from emp where 物理地址 = 0x3;
单一索引：给单个字段添加索引复合索引: 给多个字段联合起来添加1个索引主键索引：主键上会自动添加索引唯一索引：有unique约束的字段上会自动添加索引....select ename from emp where ename like '%A%';模糊查询的时候，第一个通配符使用的是%，这个时候索引是失效的。站在不同的角度去看到数据。（同一张表的数据，通过不同的角度去看待）。create view myview as select empno,ename from emp;drop view myview;注意：只有DQL语句才能以视图对象的方式创建出来。对视图进行增删改查，会影响到原表数据。（通过视图影响原表数据的，不是直接操作的原表）可以对视图进行CRUD操作。mysql> select * from myview;+-------+--------+| empno | ename |+-------+--------+| 7369 | SMITH || 7499 | ALLEN || 7521 | WARD || 7566 | JONES || 7654 | MARTIN || 7698 | BLAKE || 7782 | CLARK || 7788 | SCOTT || 7839 | KING || 7844 | TURNER || 7876 | ADAMS || 7900 | JAMES || 7902 | FORD || 7934 | MILLER |+-------+--------+create table emp_bak as select * from emp;create view myview1 as select empno,ename,sal from emp_bak;update myview1 set ename='hehe',sal=1 where empno = 7369; // 通过视图修改原表数据。delete from myview1 where empno = 7369; // 通过视图删除原表数据。视图可以隐藏表的实现细节。保密级别较高的系统，数据库只对外提供相关的视图，java程序员只对视图对象进行CRUD。将数据库当中的数据导出在windows的dos命令窗口中执行：（导出整个库）mysqldump bjpowernode>D:\bjpowernode.sql -uroot -p333在windows的dos命令窗口中执行：（导出指定数据库当中的指定表）mysqldump bjpowernode emp>D:\bjpowernode.sql -uroot –p123导入数据create database bjpowernode;use bjpowernode;source D:\bjpowernode.sql设计表的依据。按照这个三范式设计的表不会出现数据冗余。第一范式：任何一张表都应该有主键，并且每一个字段原子性不可再分。第二范式：建立在第一范式的基础之上，所有非主键字段完全依赖主键，不能产生部分依赖。多对多？三张表，关系表两个外键。t_student学生表sno(pk)sname-------------------1张三2李四3王五t_teacher 讲师表tno(pk)tname---------------------1王老师2张老师3李老师t_student_teacher_relation 学生讲师关系表id(pk)sno(fk)tno(fk)----------------------------------113211322423531633第三范式：建立在第二范式的基础之上，所有非主键字段直接依赖主键，不能产生传递依赖。一对多？两张表，多的表加外键。班级t_classcno(pk)cname--------------------------1班级12班级2学生t_studentsno(pk)snameclassno(fk)---------------------------------------------101张11102张21103张32104张42105张52提醒：在实际的开发中，以满足客户的需求为主，有的时候会拿冗余换执行速度。一对一设计有两种方案：主键共享t_user_login 用户登录表id(pk)usernamepassword--------------------------------------1zs1232ls456t_user_detail 用户详细信息表id(pk+fk)realnametel....------------------------------------------------1张三11111111112李四1111415621一对一设计有两种方案：外键唯一。t_user_login 用户登录表id(pk)usernamepassword--------------------------------------1zs1232ls456t_user_detail 用户详细信息表id(pk) realnameteluserid(fk+unique)....-----------------------------------------------------------1张三111111111122李四11114156211联系客服
