Question

我使用union all来统一多个查询，但是union all并没有为这些零值插入空行。我在网站上看了一下，根据这个问题，它应该包含它们：

这些是我的疑问：

select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 1 and SMTH > 0 )  
union all
select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 2 and SMTH > 0 )  
union all
select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 3 and SMTH > 0 )  
union all
select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 4 and SMTH > 0 )

结果：

10068
3967
895

四个独立查询：

select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 1 and SMTH > 0 )  

select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 2 and SMTH > 0 )  

select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 3 and SMTH > 0 )  

select [SMTH] from [H].[D].[T]
where Date = (SELECT MAX(DATE) from [H].[D].[T] where Data = 4 and SMTH > 0 )

结果：

所以基本上因为我认为上一个问题的答案是正确的，我做错了什么？。

谢谢！

Answer 1

哪一列为空？您确定，由于您的where子句，空值不会被删除吗？请记住，使用null的操作始终为null（除了使用IS NULL进行测试）。

Answer 2

第三个查询返回0.为什么你认为是null？这种查询的任何方式都比union：

更清晰

如果表中有主键（此查询中为id）：

select SMTH, Data, [date]
from 
    H.D.T
    inner join (
        select id, Data, max([date]) as [date]
        from H.D.T
        where Data in (1, 2, 3, 4) and SMTH > 0
        group by id, Data
    ) s on s.id = H.D.T.id
order by Data

SQL union all不包含零值

2 个答案: