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) ;

結果為:

 

表A和表B是１對多的關系   A.ID   =>   B.AID 

SELECT   ID,NAME   FROM   A   WHERE   EXIST   (SELECT   *   FROM   B   WHERE   A.ID=B.AID) 
執行結果為 
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，所以有數據

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，所以有數據

SELECT   ID,NAME   FROM   A   WHERE   EXISTS   (SELECT   *   FROM   B   WHERE   B.AID=3) 
---> SELECT   *   FROM   B   WHERE   B.AID=３無值，返回假，所以沒有數據

NOT   EXISTS   就是反過來 
SELECT   ID,NAME   FROM   A   WHERE　NOT   EXIST   (SELECT   *   FROM   B   WHERE   A.ID=B.AID) 
執行結果為 
3       A3 
=========================================================================== 
EXISTS   =   IN,意思相同不過語法上有點點區別，好像使用IN效率要差點，應該是不會執行索引的原因 
SELECT   ID,NAME   FROM   A　   WHERE　ID   IN   (SELECT   AID   FROM   B) 

NOT   EXISTS   =   NOT   IN   ,意思相同不過語法上有點點區別 
SELECT   ID,NAME   FROM   A   WHERE　ID　NOT   IN   (SELECT   AID   FROM   B)

 

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

系統要求進行SQL優化，對效率比較低的SQL進行優化，使其運行效率更高，其中要求對SQL中的部分in/not in修改為exists/not exists

 

修改方法如下：

in的SQL語句

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

修改為exists的SQL語句
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()內子查詢語句返回結果不為空說明where條件成立就會執行主sql語句，如果為空就表示where條件不成立，sql語句就不會執行。

not exists和exists相反，子查詢語句結果為空，則表示where條件成立，執行sql語句。負責不執行。

 

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'
              /*有效訂單條件*/
           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'
           /*有效訂單條件*/
        and (oi.audit_time is not null and
            oi.order_state NOT in (18, 19, 25))
  and oi.member_id = mb.member_id
這條sql是有數據的,所以not exists 在執行的時候,是沒數據的,exists是有數據的


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