如何在查询中两次使用子查询结果？

Question

我有这个查询：

UPDATE QandA SET accepted = IF ( id <> :answer_id, 0, 1 )
  WHERE col1 = ( SELECT related FROM QandA WHERE id = :answer_id ) AND
        col2 = ( SELECT related FROM QandA WHERE id = :answer_id )

如您所见，有两个相同的子查询。如何编写该子查询一次并使用其结果两次？

Answer 1

使用可以使用CROSS JOIN:

UPDATE QandA CROSS JOIN
       ( SELECT related FROM QandA WHERE id = :answer_id ) x
    SET accepted = ( id = :answer_id)
WHERE col1 =  x.related AND col2 = x.related;

请注意，我还删除了 if()。这在 MySQL 中是不必要的，其中布尔表达式在数字上下文中被视为整数。

另一种方法是使用 IN:

WHERE ( SELECT related FROM QandA WHERE id = :answer_id ) IN (col1, col2)

Answer 2

这里不需要两个子查询，一个就够了，只要确保col1 = col2就大功告成:

UPDATE QandA SET accepted = IF ( id <> :answer_id, 0, 1 )
  WHERE col1 = ( SELECT related FROM QandA WHERE id = :answer_id ) AND
        col2 = col1

如何在查询中两次使用子查询结果？

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