SQL SELECT:在“as"之前的下一个选定参数中使用“as"之后的分配值 [英] SQL SELECT: Using the assigned value after 'as' in the next selected parameter before 'as'

问题描述

我需要在下一条语句中使用 sql 中两个函数的计算值(在 'as' 关键字之后).我想使用它是因为在这种情况下计算时间减少了 x2.

I need to use the computed values (after 'as' keyword) of two functions in sql in the next statement. I want to use it because the time of computations decreases x2 in this case.

我有以下声明:

SELECT f1() as f_1, f2() as f_2, f_1 - f_2 as f1_minus_f2 FROM mytable

其中 f1(), f2() - 一些函数

where f1(), f2() - some functions

推荐答案

1) 您可以使用表达式本身(如果函数是确定性的):

1) You can use expression itself (if functions are deterministic):

SELECT f1() as f_1, f2() as f_2, f1() - f2() as f1_minus_f2 
FROM mytable

2) 使用子查询:

SELECT sub.f_1, sub.f_2, sub.f_1 - sub.f_2 as f1_minus_f2
FROM (
   SELECT f1() as f_1, f2() as f_2
   FROM mytable
) sub

您不能随意使用它的原因是 all-at-once 规则:

The reason you cannot use it as you want is all-at-once rule:

All-at-Once Operations"意味着所有表达式都在同一个逻辑查询过程阶段同时进行逻辑评估.

"All-at-Once Operations" means that all expressions in the same logical query process phase are evaluated logically at the same time.

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