条件相同时，如何对多个列使用一次CASE语句？

Question

我有这样的查询：

SELECT (CASE WHEN subject = '' THEN $subject ELSE subject END ) main_subject,
       (CASE WHEN subject = '' THEN $id ELSE id END ) main_id,
       (CASE WHEN subject = '' THEN $related ELSE related END ) main_related
FROM qanda
WHERE tag = :tag;

如您所见，条件在所有条件下都是相同的..所以我想知道，我可以改善吗？ 我的意思是在所有情况下我只能写一次CASE吗？

Answer 1

您的代码很好。 如果您确实想要，可以执行以下操作：

SELECT $subject as main_subject, $id as main_id, $related as main_related
FROM qanda
WHERE tag = :tag AND subject = ''
UNION ALL
SELECT subject as main_subject, id as main_id, related as main_related
FROM qanda
WHERE tag = :tag AND (subject <> '' or subject IS NULL)

这摆脱了CASE语句，但我认为这不是真正的简化。 但是，它确实减少了需要复制条件的次数，如果您有很多列，这可以简化。