not in null 与null运算

Not in \ in中包含有null值的列，不会排除null（因为 null != null）
Oracle not in （如果子查询返回的值或是集合有null值的话，将查询不到记录

解决办法： 子查询中限制不能游null返回值 is not null

用 not exists 代替 not in


True and null 返回 null
true or null 返回true ,
false and null返回null
false or null 返回null

null要用is进行比较如果用其它的比较运算符都返回null,
如：（1<>null返回null 1=null返回null）

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.
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)

This was on SQL Server 2005. I also found that calling set ansi_nulls off causes B to return a result.
Query A is the same as:

select 'true' where 3 = 1 or 3 = 2 or 3 = 3 or 3 = null

Since 3 = 3 is true, you get a result.
Query B is the same as:

select 'true' where 3 <> 1 and 3 <> 2 and 3 <> null

When ansi_nulls is on, 3 <> null is UNKNOWN, so the predicate evaluates to UNKNOWN, and you don't get any rows.
When ansi_nulls is off, 3 <> null is true, so the predicate evaluates to true, and you get a row.