exists oracle 的用法

CREATE TABLE `A` (
  `id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(255) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=29 DEFAULT CHARSET=latin1



CREATE TABLE `B` (
  `id` int(11) NOT NULL AUTO_INCREMENT,`AID` int(11) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=29 DEFAULT CHARSET=latin1

SELECT   ID,NAME   FROM   A   WHERE   EXISTS   (SELECT   1  FROM   B   WHERE   A.ID=B.AID) ;

结果为:

表A和表B是１对多的关系 A.ID => B.AID

SELECT ID,NAME FROM A WHERE EXIST (SELECT * FROM B WHERE A.ID=B.AID)
执行结果为
1 A1
2 A2
原因可以按照如下分析
SELECT ID,NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID=１)
---> SELECT * FROM B WHERE B.AID=１有值，返回真，所以有数据

SELECT ID,NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID=2)
---> SELECT * FROM B WHERE B.AID=２有值，返回真，所以有数据

SELECT ID,NAME FROM A WHERE EXISTS (SELECT * FROM B WHERE B.AID=3)
---> SELECT * FROM B WHERE B.AID=３无值，返回假，所以没有数据

NOT EXISTS 就是反过来
SELECT ID,NAME FROM A WHERE　NOT EXIST (SELECT * FROM B WHERE A.ID=B.AID)
执行结果为
3 A3
===========================================================================
EXISTS = IN,意思相同不过语法上有点点区别，好像使用IN效率要差点，应该是不会执行索引的原因
SELECT ID,NAME FROM A　 WHERE　ID IN (SELECT AID FROM B)

NOT EXISTS = NOT IN ,意思相同不过语法上有点点区别
SELECT ID,NAME FROM A WHERE　ID　NOT IN (SELECT AID FROM B)

===========================================================================
EXISTS：

系统要求进行sql优化，对效率比较低的sql进行优化，使其运行效率更高，其中要求对sql中的部分in/not in修改为exists/not exists

修改方法如下：

in的sql语句

SELECT id,category_id,htmlfile,title,convert(varchar(20),begintime,112) as pubtime
FROM tab_oa_pub WHERE is_check=1 and
category_id in (select id from tab_oa_pub_cate where no=‘1‘)
order by begintime desc

修改为exists的sql语句SELECT id,112) as pubtime FROM tab_oa_pub WHERE is_check=1 and exists (select id from tab_oa_pub_cate where tab_oa_pub.category_id=convert(int,no) and no=‘1‘) order by begintime desc

exists oracle 的用法

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