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[数据库]SQL求差集

数据库环境:SQL SERVER 2008R2

Sql Server有提供求集合差集的函数——EXCEPT。先看看EXCEPT的用法,

{ <query_specification> | ( <query_expression> ) } { EXCEPT }{ <query_specification> | ( <query_expression> ) }从 EXCEPT 操作数左边的查询中返回右边的查询未返回的所有非重复值。
上面是摘自MSDN对EXCEPT函数的用法介绍。在这里,我们的要求有点特别,集合B中存在多少条集合A的记录,那么,在集合A中剔除集合B中对应的记录条数。
假如A表有数据如下:
id    name
1     a
1     a
2     bB表数据如下:
id    name
1     a
3     c根据需求,B表中有一条记录和A表有重复,因此,在A表中,把该重复记录的一条去掉,
结果数据如下:
id    name
1     a
2     b
需求已经清晰了,现在开始来实现实现的方法是:分别给a表和b表的重复记录编号,
只要在b表中存在和a表编号、id、name一样的记录,即在a表进行过滤。
先准备基础数据
WITH  a     AS ( SELECT  1 AS id ,            'a' AS NAME        UNION ALL        SELECT  1 AS id ,            'a' AS NAME        UNION ALL        SELECT  2 AS id ,            'b' AS NAME        UNION ALL        SELECT  3 AS id ,            'c' AS NAME        UNION ALL        SELECT  3 AS id ,            'c' AS NAME        UNION ALL        SELECT  1 AS id ,            'a' AS NAME        UNION ALL        SELECT  4 AS id ,            'd' AS NAME       ),    b     AS ( SELECT  3 AS id ,            'c' AS NAME        UNION ALL        SELECT  1 AS id ,            'a' AS NAME        UNION ALL        SELECT  2 AS id ,            'b' AS NAME        UNION ALL        SELECT  3 AS id ,            'c' AS NAME        UNION ALL        SELECT  1 AS id ,            'a' AS NAME       )

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分别来看一下a表和b表的数据

a表       b表

第一种方式,用NOT EXISTS来实现

SELECT id ,      NAME  FROM  ( SELECT  id ,            ROW_NUMBER() OVER ( PARTITION BY id, NAME ORDER BY id ) AS nid ,            NAME       FROM   a      ) a  WHERE  NOT EXISTS ( SELECT NULL             FROM  ( SELECT  id ,                      ROW_NUMBER() OVER ( PARTITION BY id,                               NAME ORDER BY id ) AS nid ,                      NAME                 FROM   b                ) b             WHERE b.nid = a.nid                AND b.id = a.id                AND b.NAME = a.NAME )

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第二种实现方式,通过EXCEPT来实现

SELECT id ,      NAME  FROM  ( SELECT  id ,            ROW_NUMBER() OVER ( PARTITION BY id, NAME ORDER BY id ) AS nid ,            NAME       FROM   a       EXCEPT       SELECT  id ,            ROW_NUMBER() OVER ( PARTITION BY id, NAME ORDER BY id ) AS nid ,            NAME       FROM   b      ) a

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方法1和方法2本质上是一样的思路,只不过写法不同而已。

我们来看下结果

(本文完)