根据列值选择另一行

Question

如果我有下面的查询选择论坛板，但board表包含列mirror_board_id。我可以修改一个基本的选择查询，如果mirror_board_id > 0它会SELECT * FROM board WHERE board_id = mirror_board_id什么的。目前我使用2个查询来做到这一点，但是这是一个有点查询密集型...

SELECT * FROM board WHERE category_id = 7 AND board_id = 0 

更新

SELECT * FROM board WHERE category_id = 7 AND 
          (
            (
             mirror_board_id > 0 
             AND board_id = mirror_board_id 
            ) 
           OR board_id = 0 
          ) AND 
          display = 1 ORDER BY `order` ASC 

这回错了板在列表中，它实际上不是返回镜板，但原板，其中mirror_board_id > 0

Answer 1

从逻辑上讲，它是：

SELECT * 
FROM board 
WHERE (mirror_board_id > 0 and board_id = mirror_board_id) 
    or board_id = 0 

这可以更简洁地写为：

SELECT * 
FROM board 
WHERE board_id = coalesce(mirror_board_id, 0)