如何通过另一个计算列使用计算列 [英] How to use a calculated column by another calculated column

问题描述

我在 Sqlite 中有一个涉及复杂列计算的查询，比方说:

SELECT 1+1 AS a;

我想将此计算选择为 a，但我还需要将其用作另一个计算的组件:

SELECT 1+1 AS a, a+2 AS b;

不幸的是，这会产生错误:

错误:没有这样的列:a

我知道我可以简单地为 b 再次重复计算:

SELECT 1+1 AS a, 1+1+2 AS b;

但是假设 1+1 是一些复杂且昂贵的操作，有什么方法可以让我稍后在 SELECT 中引用它而不必重新计算它?

解决方案

您需要使用子查询.

SELECT c.d AS a, c.d + 2 AS b发件人(SELECT 1+1 AS d) c

结果

<前>||乙 |---------|2 |4 |

I have a query in Sqlite that involves a complicated column calculation, let's say:

SELECT 1+1 AS a;

I want to have this calculation selected as a, but I also need to use it as a component of another calculation:

SELECT 1+1 AS a, a+2 AS b;

Unfortunately this produces the error:

Error: no such column: a

I know that I could simply repeat the calculation again for b:

SELECT 1+1 AS a, 1+1+2 AS b;

But assuming 1+1 is some complicated and expensive operation, is there any way that I can reference it later in the SELECT without having to recalculate it?

解决方案

You need to use a sub-query.

SELECT c.d AS a, c.d + 2 AS b
FROM
  (SELECT 1+1 AS d) c

Result

| a | b |
---------
| 2 | 4 |

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