推荐回答(4个)
1.exist,not exist一般都是与子查询一起使用. In可以与子查询一起使用,也可以直接in (a,b.....)
2.exist会针对子查询的表使用索引. not exist会对主子查询都会使用索引. in与子查询一起使用的时候,只能针对主查询使用索引. not in则不会使用任何索引. 注意,一直以来认为exists比in效率高的说法是不准确的。
in 是把外表和内表作hash 连接,而exists是对外表作loop循环,每次loop循环再对内表进行查询。
如果查询的两个表大小相当,那么用in和exists差别不大。
如果两个表中一个较小,一个是大表,则子查询表大的用exists,子查询表小的用in:
例如:表A(小表),表B(大表)1:select * from A where cc in (select cc from B)
效率低,用到了A表上cc列的索引;select * from A where exists(select cc from B where cc=A.cc)
效率高,用到了B表上cc列的索引。
相反的2:select * from B where cc in (select cc from A)
效率高,用到了B表上cc列的索引;select * from B where exists(select cc from A where cc=B.cc)
效率低,用到了A表上cc列的索引。
not in 和not exists如果查询语句使用了not in 那么内外表都进行全表扫描,没有用到索引;而not extsts 的子查询依然能用到表上的索引。所以无论那个表大,用not exists都比not in要快。
3.exist与in都可以实现一个目的.二者都可以用来过滤数据.
示例:
select count(1) from t1;--160W
select count(1) from t2; --90W
SELECT count(1)
FROM t1 a
WHERE EXISTS (SELECT accountid
FROM t2 b
WHERE a.keyid = b.keyid AND a.ideaid = b.ideaid);--主大子小,不适合使用exist,因为exist只会利用子表t2的复合索引keyid+ideaid,而子表内容要小与主表,主表由于无法使用索引,查询效率低下.
select count(1) from t1 a where accountid in (SELECT accountid
FROM t2 b
WHERE a.keyid = b.keyid AND a.ideaid = b.ideaid);--主大子小,适合用in,因为in只会使用主表t1里面的复合主键keyid-ideaid,在主表大于子表的情况下,会很好的利用主表的索引.
--后二条sql的执行结果都是一样的.说明exist与in在用法上可以达到一个目的,不同的地方是
--1.性能的考虑此时就按子表大主表小用exist,子表小主表大用in的原则就可以.
--2.写法的不同, exist的where条件是: "...... where exist (..... where a.id=b.id)"
--in的where条件是: " ...... where id in ( select id .... where a.id=b.id)"
4. exist的原理:
exists做为where 条件时,是先对where 前的主查询询进行查询,然后用主查询的结果一个一个的代入exists的查询进行判断,如果为真则输出当前这一条主查询的结果,否则不输出
比如
如下:
表A
ID NAME
1 A1
2 A2
3 A3
表B
ID AID NAME
1 1 B1
2 2 B2
3 2 B3
表A和表B是一对多的关系 A.ID --> B.AID
SELECT ID , NAME FROM A WHERE EXISTS (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无值返回真所以没有数据
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
5. in 与 =的区别
select name from student where name in ('zhang','wang','li','zhao');
与
select name from student where name='zhang' or name='li' or name='wang' or name='zhao'
的结果是相同的。
in的字段也可以与其它字段建复合索引.
比如
T1包含下面key, accountd,groupid.
SELECT *
FROM T1 a
WHERE a.groupid = 2001
AND a.accountid = 1001
AND a.key IN ('abc', 'def', 'ala');
--上面的sql可以将accountid,key建成复合索引.
在用in的子查询中只能返回一列值,比如 WHERE 成绩 in(select 成绩,学号 FROM 成绩表) 这种用法是不允许的。exists是一个函数,是根据子查询有无记录返回判断条件的成立,如果子查询有记录返回,那么条件成立,否则不成立,所以子查询中给出列名无意义的,因此子查询通常用*返回所有列。还有,就是in的子查询不需要关联外部表,exists子查询通常要关联外部表。
在SQL中,同样的查询很多情况下,既可以用in的用法来写,也可以用exists来写。in写法比exists的写法要容易读,好理解,但是当数据量大的时候,exists的效率要远高于in的写法。
in是集合查询,查询当前记录的属性值是否在一个集合中(集合或子查询)
exist是存在查询,查询与当前记录关联的集合(子查询)是否存在记录
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