如何在另一个结果列的expression式中重用结果列

例：

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

当然，这是无效的（至less在Postgres），但如何在不重写子查询两次查询中实现相同的？

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

你可以像这样重复使用这个查询：

 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 

这里有一点要注意。 第一种方法更具可读性和可重用性，但第二种方法可能会更快，因为数据库可能会select更好的scheme。

Answer 3

也许sql“with”子句可以提供帮助，如http://orafaq.com/node/1879 （其他数据库如Postgres也是这样做的，不只是oracle）。

干杯! 安德烈

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，减less了我的查询阅读开销90％….

所以，而不是复制相关的查询多次从子查询中检索多个列，我只是用concat所有的值我想返回到一个逗号分隔的varchar，然后在应用程序中再次展开它们…

所以，而不是

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

我现在呢

 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“标量”值，但只需要执行相关子查询一次而不是三次。

Answer 5

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

Answer 6

我认为以下将起作用：

 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); 

没有在动物身上testing – 你会成为第一！ 🙂

分享和享受。

Answer 7

你可以像这样使用用户定义的variables

 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

使用交叉申请或外部申请。

 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

这是相当古老，但我遇到了这个问题，看到这个post，但没有设法解决我的问题，使用给定的答案，所以我最终到达这个解决scheme：

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，然后使用如下的字段：

 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，如果这是一个不好的方式做事情，但performance明智，似乎没关系，我查询超过224,000logging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。