如何在表达式重用结果列另一个结果列

Question

实施例：

SELECT 
    (SELECT SUM(...) FROM ...) as turnover, 
    (SELECT SUM(...) FROM ...) as cost, 
    turnover - cost as profit 

当然这是无效的（至少在Postgres的），但如何实现在一个查询中的相同，不重写子查询两次？

Answer 1

像这样：

SELECT 
    turnover, 
    cost, 
    turnover - cost as profit 
from (
    (SELECT SUM(...) FROM ...) as turnover, 
    (SELECT SUM(...) FROM ...) as cost 
    ) as partial_sums 

Answer 2

SELECT turnover, cost, turnover - cost 
FROM 
(
SELECT 
(SELECT ...) as turnover, 
(SELECT ...) as cost 
) as Temp 

Answer 3

我觉得下面的工作：

SELECT turnover, cost, turnover-cost as profit FROM 
    (SELECT 1 AS FAKE_KEY, SUM(a_field) AS TURNOVER FROM some_table) a 
INNER JOIN 
    (SELECT 1 AS FAKE_KEY, SUM(a_nother_field) AS COST FROM some_other_table) b 
USING (FAKE_KEY); 

在动物测试 - 你会先来！ :-)

分享和享受。

Answer 4

其实我在这方面做了很多工作，打了很多砖墙，但最后想出了一个答案 - 更多的是黑客 - 但它工作得很好，并减少了我的查询阅读开销90％.. ..

因此，而不是重复相关的查询多次从子查询中检索多个列，我只是使用concat所有值我想返回到一个逗号分隔varchar，然后在应用程序中再次展开它们。 。

所以不是

select a,b, 
(select x from bigcorrelatedsubquery) as x, 
(select y from bigcorrelatedsubquery) as y, 
(select z from bigcorrelatedsubquery) as z 
from outertable 

我现在做

select a,b, 
(select convert(varchar,x)+','+convert(varchar,x)+','+convert(varchar,x)+',' 
from bigcorrelatedsubquery) from bigcorrelatedquery) as xyz 
from outertable 
group by country 

我现在有三个相关的“标”我需要的，但只有执行相关子查询一次，而不是三次值。

Answer 5

也许SQL“与”条款可以帮助，因为这里介绍http://orafaq.com/node/1879（其他数据库如Postgres里做得一样好，而不仅仅是甲骨文）。

Answer 6

可以重用这样的查询：

WITH 
    TURNOVER AS (
    SELECT SUM(...) FROM ...) 
), 
    COST AS(
    SELECT SUM(...) FROM ... 
) 

SELECT * 
FROM(
SELECT 
    TURNOVER.sum as SUM_TURNOVER 
FROM 
TURNOVER,COST 
WHERE .... 
) AS a 

这相当于：

SELECT * 
FROM(
SELECT 
    TURNOVER.sum as SUM_TURNOVER 
FROM 
(
    SELECT SUM(...) FROM ...) 
)AS TURNOVER, 
(
    SELECT SUM(...) FROM ... 
)AS COST 
WHERE .... 
) AS a 

有一点要注意这里。第一种方法更具可读性和可重用性，但第二种方法可能更快，因为数据库可能会为其选择更好的计划。

Answer 7

您可以使用用户定义的变量这样

SELECT 
    @turnover := (SELECT SUM(...) FROM ...), 
    @cost := (SELECT SUM(...) FROM ...), 
    @turnover - @cost as profit 

http://dev.mysql.com/doc/refman/5.7/en/user-variables.html

Answer 8

使用交叉应用或外适用。

SELECT 
    Calc1.turnover, 
    Calc2.cost, 
    Calc3.profit 
from 
    cross apply ((SELECT SUM(...) as turnover FROM ...)) as Calc1 
    cross apply ((SELECT SUM(...) as cost FROM ...)) as Calc2 

    /* 
    Note there is no from Clause in Calc 3 below. 
    This is how you can "stack" formulas like in excel. 
    You can return any number of columns, not just one. 
    */ 
    cross apply (select Calc1.turnover - Calc2.cost as profit) as Calc3 

Answer 9

这是很老，但我遇到了这个问题，看到这篇文章但没有管理使用给出的答案，所以我最终抵达这一解决方案，以解决我的问题：

如果您的查询是：

SELECT 
    (SELECT SUM(...) FROM ...) as turnover, 
    (SELECT SUM(...) FROM ...) as cost, 
    turnover - cost as profit 

，你可以把它变成一个子查询，然后使用领域，如：

SELECT *,(myFields.turnover-myFields.cost) as profit 
FROM 
(  
SELECT 
     (SELECT SUM(...) FROM ...) as turnover, 
     (SELECT SUM(...) FROM ...) as cost 

) as myFields 

我不完全确定如果这是一种不好的方式做事情，但性能明智的看起来好像我查询224,000记录花了1.5秒 不知道它后来变成了由DB相同的子查询的2倍。