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,
  `name` varchar(255) 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, category_id, htmlfile, title, convert(varchar(20),begintime,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表示()内子查询语句返回结果不为空说明where条件成立就会执行主sql语句，如果为空就表示where条件不成立，sql语句就不会执行。

not exists和exists相反，子查询语句结果为空，则表示where条件成立，执行sql语句。负责不执行。

select nvl(count(distinct o.member_id), 0) as newMembers
  from order_info o, member mb
 where o.member_id = mb.member_id
   and o.is_delete = 'N'
   and mb.is_delete = 'N'
   and   not exists (select oi.member_id
          from order_info oi, member mb
         where oi.is_delete = 'N'
              /*有效订单条件*/
           and (oi.audit_time is not null and
               oi.order_state NOT in (18, 19, 25))
     and oi.member_id = mb.member_id)

select oi.member_id
       from order_info oi, member mb
      where oi.is_delete = 'N'
           /*有效订单条件*/
        and (oi.audit_time is not null and
            oi.order_state NOT in (18, 19, 25))
  and oi.member_id = mb.member_id
这条sql是有数据的,所以not exists 在执行的时候,是没数据的,exists是有数据的

https://www.iteye.com/blog/yangzhonglei-699673