Question

我使用Union来合并两个SQL查询，但结果不同于两个不同查询的组合。下面是我的SQL代码：

(SELECT CONCAT(Name, '(', LEFT(Occupation, 1), ')')
FROM OCCUPATIONS
ORDER BY Name ASC)
UNION
(SELECT CONCAT('There are a total of ', COUNT(Occupation), ' ', 
LOWER(Occupation), 's.')
FROM OCCUPATIONS
GROUP BY Occupation
ORDER BY Occupation ASC)

如果我只运行查询的前半部分，我会得到以下结果：

Aamina(D) 
Ashley(P) 
Belvet(P) 
Britney(P) 
Christeen(S) 
Eve(A) 
Jane(S) 
Jennifer(A) 
Jenny(S) 
Julia(D) 
Ketty(A)  
Kristeen(S) 
Maria(P) 
Meera(P) 
Naomi(P) 
Priya(D) 
Priyanka(P) 
Samantha(A)

如果我只运行查询的后半部分，我会得到以下结果：

There are a total of 4 actors. 
There are a total of 3 doctors. 
There are a total of 7 professors. 
There are a total of 4 singers.

以上两个结果均按预期顺序排列。但是，如果我运行所有查询，我会得到以下结果：

Ashley(P) 
Samantha(A) 
Julia(D) 
Britney(P) 
Maria(P) 
Meera(P) 
Priya(D) 
Priyanka(P) 
Jennifer(A) 
Ketty(A) 
Belvet(P) 
Naomi(P) 
Jane(S) 
Jenny(S) 
Kristeen(S) 
Christeen(S) 
Eve(A) 
Aamina(D) 
There are a total of 4 actors. 
There are a total of 3 doctors. 
There are a total of 7 professors. 
There are a total of 4 singers.

正如您可能注意到的那样，上半部的顺序被搞砸了。有谁知道为什么？ Union如何与写两个单独的SQL查询不同？谢谢！

Answer 1

订单没有“搞砸”。整个查询没有order by，仅适用于子查询。订单不会保留。您正在使用UNION，这会删除重复项。

执行此查询的安全方法是：

select str
from ((select concat(Name, '(', LEFT(Occupation, 1), ')') as str, 1 as which
       from OCCUPATIONS
      ) union all
      (select concat('There are a total of ', COUNT(Occupation), ' ', 
lower(Occupation), 's.') as str, 2 as which
       from OCCUPATIONS
       group by occupation
      )
     ) o
order by which, str

SQL A Union B的结果与A + B不同？

1 个答案: