sql语句是否相等？

问题描述

我只是想弄清楚这些语句是否相等：

SELECT *
FROM houses LEFT JOIN tenants on houses.id= tenants.id
WHERE houses.id = 005 and tenants.address = '1 chavvy road'

SELECT *
FROM houses LEFT JOIN tenants on houses.id= tenants.id and tenants.address='1 chavvy road'
WHERE houses.id = 005

我创建了一个虚拟数据库，看起来它们是等效的。当这些都不相等时，谁能想到一个例子？

我的理解是，这些语句基本上给出相同的结果。在我看来，代码1在联接之后对租户地址进行过滤，而代码2在联接内进行过滤。

解决方法

如果它是INNER JOIN，则查询将是等效的。但是由于这是LEFT JOIN，所以不是。假设tenants持有元组id: 005,address: 'bla'。第一个查询检查ID是否匹配，执行连接，然后查看地址不匹配，并省略该行。第二个查询不加入，因为地址不匹配，但不会省略该行，而是为NULL的所有列返回tenants！

WITH houses (id) AS (
    SELECT *
    FROM (VALUES (5)) as a
),tenants (id,address) AS (
    SELECT *
    FROM (VALUES (5,'bla')
         ) as tenants
)
SELECT *
FROM houses LEFT JOIN tenants ON houses.id = tenants.id
WHERE houses.id = 005 and tenants.address = '1 chavvy road';

WITH houses (id) AS (
    SELECT *
    FROM (VALUES (5)) as a
),'bla')
         ) as tenants
)
SELECT *
FROM houses LEFT JOIN tenants ON houses.id = tenants.id and tenants.address = '1 chavvy road'
WHERE houses.id = 005;