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，我可以改进吗？我的意思是我可以为所有情况只写一次CASE吗？

Answer 1

你的代码很好。如果你真的想，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)

这摆脱了CASE语句，但我不认为这是一个真正的简化。但是，它确实减少了条件需要复制的次数，如果你有很多列，这可以简化。

当条件相同时，如何对多列使用CASE语句一次？

1 个答案: