SQL if语句

Question

如何在sql中使用if语句？我想做这样的事情：

SELECT DISTINCT exchange.id userid, exchange.name username, exchange.lastactivity     lastactivity, exchange.id avatar, exchange.id link, cometchat_status.message, cometchat_status.status
FROM buddy
if( buddy.penpalA = 887){
      JOIN exchange ON buddy.penpalA = exchange.id
}ELSE{
      JOIN exchange ON buddy.penpalB = exchange.id
}
LEFT JOIN cometchat_status ON exchange.id = cometchat_status.userid
if( buddy.penpalA = 887){
      WHERE buddy.penpalB =887
}ELSE{
      WHERE buddy.penpalA =887
}    

ORDER BY username ASC 

Answer 1

只需修改您的WHERE和ON。请注意，ANDed比较周围的段落是不必要的；我这样做只是为了澄清正在发生的事情。

SELECT DISTINCT exchange.id userid, exchange.name username,
                exchange.lastactivity lastactivity, exchange.id avatar,
                exchange.id link, cometchat_status.message,
                cometchat_status.status
FROM buddy
JOIN exchange ON (buddy.penpalA = exchange.id AND buddy.penpalA = 887)
              OR (buddy.penpalB = exchange.id AND buddy.penpalA <> 887 AND buddy.penpalB = 887)
LEFT JOIN cometchat_status ON exchange.id = cometchat_status.userid
WHERE buddy.penpalA = 887 OR buddy.penpalB = 887
ORDER BY username ASC

更新：

我更改了上面的代码，以便在penpalB为887时也加入，根据我对您的评论的理解。

Answer 2

使用Transact-SQL，您可以构造一个IF-ELSE，如下所示：

IF a > b
BEGIN
   // do something
END
ELSE
   // do something else