postgresql - ORDER BY 子句中常​​规字段和计算字段的乘积

Question

以下查询工作正常:

SELECT a, b, c,
(SELECT COUNT(*) AS COUNT FROM table_b WHERE field_a = p.a) AS d
FROM table_a AS p
ORDER BY b DESC

这也有效:

SELECT a, b, c,
(SELECT COUNT(*) AS COUNT FROM table_b WHERE field_a = p.a) AS d
FROM table_a AS p
ORDER BY d DESC

但是下面会产生一个错误； “d”列不存在错误:

SELECT a, b, c,
(SELECT COUNT(*) AS COUNT FROM table_b WHERE field_a = p.a) AS d
FROM table_a AS p
ORDER BY (b * d) DESC

以上三个查询之间的唯一区别是 ORDER BY 子句。在前两个查询中，结果按 b 字段或动态 d 字段排序。在最后一个查询中，结果(应该)按 b 乘以 d 的乘积排序。

为什么在最后一个查询中，PostgreSQL 说 d 不存在，而在第二个查询中却可以毫无问题地找到它？

Answer 1

order by clause 中的任意表达式只能由输入列组成:

Each expression can be the name or ordinal number of an output column (SELECT list item), or it can be an arbitrary expression formed from input-column values.

您需要对其进行子查询:

select *
from (select 1 as a, 2 as b) s
order by a * b