sql-server - SQL Not Exists 包含空值

Question

我有两个 SQL Server 表:

表 1

Id name   description     version
----------------------------------
1  Book1  Book 1 Title    v1
2  Book2  Book 2 Title    v2
3  Book3  Book 3 Title    NULL
4  Book5  Book 5 Title    v3

表 2

Id name   description     version
----------------------------------
1  Book1  Book 1 Title    v1
2  Book2  Book 2 Title    v2
3  Book3  Book 3 Title    NULL
4  Book4  Book 4 Title    NULL
5  Book5  Book 5 Title    NULL

我想从表 2 中选择表 1 中不存在的所有数据，这样我就可以将它们插入另一个表。

这是 SQL 查询:

SELECT t2.name, t2.description, t2.version
FROM Table2 AS t2
WHERE  
    NOT EXISTS (SELECT t1.name, t1.description, t1.version 
                FROM Table1 as t1 
                WHERE t2.name = t1.name 
                  AND t2.description = t1.description  
                  AND t2.version = t1.version)

预期的结果是这样的:

Id name   description     version
-----------------------------------
4  Book4  Book 4 Title    NULL
5  Book5  Book 5 Title    NULL

但我得到的是这个:

Id name   description     version
---------------------------------
3  Book3  Book 3 Title    NULL
4  Book4  Book 4 Title    NULL

为什么在我的 NOT EXIST 语句中不计算空值并显示？

Answer 1

NULL 值与任何值比较时将返回 FALSE。您将需要使用 IS NULL 来处理

select t2.name, t2.description, t2.version
from   Table2 as t2
WHERE  NOT EXISTS 
       ( 
            SELECT *
            FROM   Table1 as t1 
            WHERE  t2.name        = t1.name 
            AND    t2.description = t1.description 
            AND    (
                       t2.version     = t1.version
                   OR  (t2.version IS NULL AND t1.version IS NULL)
                   )
       )