基于SQL求集合的交、并、补

对数据库查询结果集求补（差）、交、并集，可利用SQL的EXCEPT（补集）、INTERSECT（交集）和UNION（并集）关键词，其中Mysql仅支持UNION，但可以利用SQL实现EXCEPT、INTERSECT的功能。

作者接下来将给出实验用表以及具体的SQL脚本语句。

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实验用表

为进行实验，作者在SQL Server数据库下共建立了t1、t2两张表，两张表除主键id外均有个两字段stu_id和teacher_id。

t1表

id	stu_id	teacher_id
1	S0001	T01
2	S0002	T01
3	S0003	T02
4	S0004	T02
5	S0005	T03
6	S0002	T04

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t2表

id	stu_id	teacher_id
1	S0002	T05
2	S0003	T03
3	S0006	T02
4	S0007	T04
5	S0005	T03

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SQL脚本语句

/*求补（差）集demo1，一个字段*/
SELECT a.stu_id FROM t1 a
EXCEPT
SELECT a.stu_id FROM t2 a;
/*和上面等效，可用于Mysql*/
SELECT a.stu_id FROM t1 a
WHERE a.stu_id NOT IN
(SELECT a.stu_id FROM t2 a);

/*求补（差）集demo2，多个字段求差集*/
SELECT a.stu_id,a.teacher_id FROM t1 a
WHERE a.stu_id NOT IN
(SELECT a.stu_id FROM t2 a)
AND a.teacher_id NOT IN
(SELECT a.teacher_id FROM t2 a);

/*求交集demo1，多个字段求交集*/
SELECT a.stu_id,a.teacher_id FROM t1 a
INTERSECT
SELECT a.stu_id,a.teacher_id FROM t2 a;
/*和上面等效，可用于Mysql*/
SELECT a.stu_id,a.teacher_id FROM t1 a
WHERE a.stu_id+a.teacher_id IN
(SELECT a.stu_id+a.teacher_id FROM t2 a);

/*求交集demo2*/
/*下面语句把交集中重复出现的去掉*/
SELECT a.stu_id FROM t1 a
INTERSECT
SELECT a.stu_id FROM t2 a;

/*下面三个语句不会把重复的去掉，可使用DISTINCT去重*/
SELECT a.stu_id FROM t1 a
WHERE a.stu_id IN
(SELECT a.stu_id FROM t2 a);

SELECT a.stu_id FROM t1 a,t2 b
WHERE a.stu_id=b.stu_id;

SELECT a.stu_id FROM t1 a
WHERE EXISTS
(SELECT * FROM t2 b WHERE a.stu_id=b.stu_id);

/*求并集demo*/
/*不会去重*/
SELECT a.stu_id FROM t1 a
UNION ALL
SELECT a.stu_id FROM t2 a;
/*去重，Mysql中可以在UNION后加DISTINCT*/
SELECT a.stu_id FROM t1 a
UNION
SELECT a.stu_id FROM t2 a;