Question

 $sql = "select u.Id, u.username, u.email, (NOW()-u.authenticationTime) as authenticateTimeDifference, u.IP, 
          f.providerId, f.requestId, f.status, u.port 
       from friends f
       left join users u on 
          u.Id = if ( f.providerId = ".$userId.", f.requestId, f.providerId ) 
       where (f.providerId = ".$userId." and f.status=".USER_APPROVED.")  or 
           f.requestId = ".$userId." ORDER BY u.username ASC";

我不明白这里的if含义，与where

一起使用

// providerId and requestId is Id of  a friend pair,
// providerId is the Id of making first friend request
// requestId is the Id of the friend approved the friend request made by providerId

// fetching friends, 
// left join expression is a bit different, 
//  it is required to fetch the friend, not the users itself

谁能为我解释一下？ thanx

Answer 1

只有当条件f.providerId = ".$userId."为真时，才能通过requestId在好友和用户之间建立联接。如果不是，则由providerId进行连接。

我看到$userId是一个你传递给查询的php变量，所以可能的想法是检查活动用户是否是提供者，然后获取与请求相关联的行而不是朋友表中的提供者。

关于If函数的文档：http://dev.mysql.com/doc/refman/5.1/en/control-flow-functions.html#function_if

mysql我不明白这里的含义

1 个答案: