Mysql Information_schema

 

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。Mysql的INFORMATION_SCHEMA数据库包含了一些表和视图，提供了访问数据库元数据的方式，这台MySQL服务器上，到底有哪些数据库、各个数据库有哪些表，每张表的字段类型是什么，各个数据库要什么权限才能访问，等等信息都保存在information_schema表里面. 让我们来看看几个使用这个数据库的例子

http://hudeyong926.iteye.com/blog/2380734

 

查询数据库dj214中表数据超过 1000 行的表

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

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

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

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

SELECT table_name, table_type, engine  FROM INFORMATION_SCHEMA.TABLES WHERE table_schema = 'dj214' AND table_type='BASE TABLE'

列出指定数据库下指定表的表结构

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 ]

	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 ]

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