是否保证保留子查询中的顺序? [英] Is order in a subquery guaranteed to be preserved?

问题描述

我特别想知道 PostgreSQL.给定以下人为的示例:

I am wondering in particular about PostgreSQL. Given the following contrived example:

SELECT name FROM
  (SELECT name FROM people WHERE age >= 18 ORDER BY age DESC) p
LIMIT 10

外部查询返回的名称是否保证与内部查询的顺序一致?

Are the names returned from the outer query guaranteed to be be in the order they were for the inner query?

推荐答案

不，在外层查询中按顺序排列:

No, put the order by in the outer query:

SELECT name FROM
  (SELECT name, age FROM people WHERE age >= 18) p
ORDER BY p.age DESC
LIMIT 10

内部(子)查询返回结果集.如果您将 order by 放在那里，则从内部(子)查询传递到外部查询的中间结果集保证按照您指定的方式排序，但在外部查询中没有 order by，结果通过处理该内部查询结果集生成的 -set 不保证以任何方式排序.

The inner (sub) query returns a result-set. If you put the order by there, then the intermediate result-set passed from the inner (sub) query, to the outer query, is guaranteed to be ordered the way you designate, but without an order by in the outer query, the result-set generated by processing that inner query result-set, is not guaranteed to be sorted in any way.

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