问题描述
当我得到不同的记录计数时出现了这个问题，我认为是相同的查询，一个使用 not in where 约束，另一个使用 left join.not in 约束中的表有一个空值(坏数据)，这导致该查询返回 0 条记录.我有点理解为什么，但我可以借助一些帮助来完全掌握这个概念.
This issue came up when I got different records counts for what I thought were identical queries one using a not in where constraint and the other a left join. The table in the not in constraint had one null value (bad data) which caused that query to return a count of 0 records. I sort of understand why but I could use some help fully grasping the concept.
简单地说，为什么查询 A 返回结果而 B 没有?
To state it simply, why does query A return a result but B doesn't?
A: select 'true' where 3 in (1, 2, 3, null)
B: select 'true' where 3 not in (1, 2, null)
这是在 SQL Server 2005 上.我还发现调用 set ansi_nulls off 会导致 B 返回结果.
This was on SQL Server 2005. I also found that calling set ansi_nulls off causes B to return a result.
推荐答案
Query A 同:
select 'true' where 3 = 1 or 3 = 2 or 3 = 3 or 3 = null
因为 3 = 3 为真，所以你得到了结果.
Since 3 = 3 is true, you get a result.
查询 B 与以下内容相同:
Query B is the same as:
select 'true' where 3 <> 1 and 3 <> 2 and 3 <> null
当 ansi_nulls 开启时，3 <>null 是 UNKNOWN，因此谓词的计算结果为 UNKNOWN，并且您不会得到任何行.
When ansi_nulls is on, 3 <> null is UNKNOWN, so the predicate evaluates to UNKNOWN, and you don't get any rows.
当 ansi_nulls 关闭时，3 <>null 为真，因此谓词的计算结果为真，并且您得到一行.
When ansi_nulls is off, 3 <> null is true, so the predicate evaluates to true, and you get a row.
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