MySQL不支持INTERSECT和MINUS，及其替代方法

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#join #JOIN #Join #mysql #MySQL #Mysql #MYSQL #MySql #mysqL #table #transformation #tuples

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本文介绍如何在MySQL中实现INTERSECT和MINUS操作。对于INTERSECT，可以通过内连接来找到两个表中相同的记录；而对于MINUS，则可以通过子查询或者左连接并结合NULL判断来找出一个表中有而另一个表中没有的记录。

Doing INTERSECT and MINUS in MySQL

By Carsten | October 3, 2005

Doing an INTERSECT

An INTERSECT is simply an inner join where we compare the tuples of one table with those of the other, and select those that appear in both while weeding out duplicates. So

SELECT member_id, name FROM a
INTERSECT
SELECT member_id, name FROM b

can simply be rewritten to

SELECT a.member_id, a.name
FROM a INNER JOIN b
USING (member_id, name)

Performing a MINUS
To transform the statement

SELECT member_id, name FROM a
MINUS
SELECT member_id, name FROM b

into something that MySQL can process, we can utilize subqueries (available from MySQL 4.1 onward). The easy-to-understand transformation is:

SELECT DISTINCT member_id, name
FROM a
WHERE (member_id, name) NOT IN
(SELECT member_id, name FROM table2);

Of course, to any long-time MySQL user, this is immediately obvious as the classical use-left-join-to-find-what-isn’t-in-the-other-table:

SELECT DISTINCT a.member_id, a.name
FROM a LEFT JOIN b USING (member_id, name)
WHERE b.member_id IS NULL