php - 当条件相同时，如何对多列使用一次 CASE 语句？

Question

我有一个这样的查询:

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，所有条件下的条件都是相同的..所以我想知道，我可以改进吗？我的意思是我可以为所有情况只编写一次 CASE 吗？

Answer 1

你的代码没问题。如果你真的愿意，你可以这样做:

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)

这消除了 CASE 语句，但我认为这并不是真正的简化。但是，它确实减少了需要复制条件的次数，如果您有很多列，这可以是一种简化。