从MySQL 5 开始 , 你可以看到多了一个系统数据库 information_schema . information_schema 存贮了其他所有数据库的信息。 information_schema是一个虚拟数据库，并不物理存在，在 select 的时候，从其他数据库获取相应的信息。 让我们来看看几个使用这个数据库的例子
查询数据库dj214中表数据超过 1000 行的表
Java代码
select concat(table_schema,'.',table_name) as table_name,table_rows
from information_schema.tables where table_rows > 1000 and table_schema = 'dj214' order by table_rows desc;
查询数据库dj214 中所有没有主键的表
Java代码
SELECT CONCAT(t.table_schema,".",t.table_name) as table_name
FROM information_schema.TABLES t
LEFT JOIN information_schema.TABLE_CONSTRAINTS tc
ON t.table_schema = tc.table_schema
AND t.table_name = tc.table_name
AND tc.constraint_type = 'PRIMARY KEY'
WHERE tc.constraint_name IS NULL
AND t.table_type = 'BASE TABLE' AND t.table_schema = 'dj214' ;
查询所有数据库中10张最大表
Java代码
SELECT concat(table_schema,'.',table_name) 表名称,
concat(round(data_length/(1024*1024),2),'M') 表大小
FROM information_schema.TABLES
ORDER BY data_length DESC LIMIT 10;
查看MYSQL数据库下所有的数据库
Java代码
SELECT SCHEMA_NAME AS 'database' FROM INFORMATION_SCHEMA.SCHEMATA LIMIT 0 , 30
列出指定数据库中的所有表名称
Java代码
SELECT table_name FROM INFORMATION_SCHEMA.TABLES WHERE table_schema = 'dj214'
列出指定数据库下指定表的表结构
Java代码
SELECT COLUMN_NAME, DATA_TYPE, IS_NULLABLE, COLUMN_DEFAULT
FROM INFORMATION_SCHEMA.COLUMNS WHERE table_name = 'systemlog' AND table_schema = 'dj214'
一段MYSQL存储过程 [ 删除指定库中所有的空表 ]
Java代码
begin
/*局部变量的定义,默认值为空 */
declare tmpName varchar(200) default '' ;
/*定义游标*/
DECLARE reslutList Cursor FOR select table_name from information_schema.tables where table_rows <1 and table_schema = 'sz8_news' order by table_rows desc;
declare CONTINUE HANDLER FOR SQLSTATE '02000' SET tmpname = null;
OPEN reslutList;/*打开游标*/
FETCH reslutList into tmpname; -- 取数据
/* 循环体 */
WHILE ( tmpname is not null) DO
set @sql = concat('drop table sz8_news.',tmpname,";");
PREPARE stmt1 FROM @sql ;
EXECUTE stmt1 ;
DEALLOCATE PREPARE stmt1;
/*游标向下走一步*/
FETCH reslutList INTO tmpname;
END WHILE;
CLOSE reslutList; /*关闭游标*/
end
一段 MYSQL存储过程 [ 删除指定库下所有表中的空列，即表中的任何一条记录该列都没有值 ]
Java代码
BEGIN
DECLARE done INT DEFAULT 0;
DECLARE cTbl varchar(64);
DECLARE cCol varchar(64);
DECLARE cur1 CURSOR FOR
select TABLE_NAME,COLUMN_NAME from INFORMATION_SCHEMA.COLUMNS
where TABLE_SCHEMA='sz8_news' and IS_NULLABLE='YES' order by TABLE_NAME;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
set @sqlDrop='';
OPEN cur1;
FETCH cur1 INTO cTbl, cCol;/*得到表名及列名*/
WHILE done = 0 DO
set @x=0;
/*主要改进了这里把空值也纳入判断条件中去，即如果字段为null或空*/
set @sql=concat('select 1 into @x from ',cTbl,' where ',cCol,' is not null and ',cCol,' !="" limit 1');
PREPARE stmt1 FROM @sql;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
if @x=0 then
set @sqlDrop=concat('alter table `',cTbl,'` drop COLUMN`',cCol,'`;');
PREPARE stmt1 FROM @sqlDrop;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
end if ;
set done = 0;
FETCH cur1 INTO cTbl, cCol;
END WHILE;
CLOSE cur1;
END
