SQL union全部不包含零值

Question

我使用union all來聯合多個查詢，但union all沒有爲這些零值插入空行。我參加了現場一看，並根據這個問題，它應該包括他們：

mysql union with NULL fields

這是我的查詢：

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) 

結果：

10068 
3967 
895 

四個獨立查詢：

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) 

結果：

10068 
3967 
0 
895 

所以基本上，因爲我認爲，從以前的問題的答案是正確的，我在做什麼錯？

謝謝！

Answer 1

哪一列爲空？ 您確定，由於您的where子句，空值不會被刪除嗎？ 請記住，使用null的操作總是導致null（除了使用IS NULL進行測試）。

Answer 2

第三個查詢返回0.爲什麼你認爲這是null？無論如何這個查詢比union清潔：

如果表中有一個主鍵（id在查詢）：

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