sql+语句研究

select u.Id, u.username, (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 = ".$userId." select Id from users where username = '".$username."' limit 1 insert into users(username, password, email) values ('".$username." ', '".$password."', '".$email."') select Id from users where username='".$friendUserName."' limit 1" insert into friends(providerId, requestId, status) values(".$userId. ", ".$requestId.", ".USER_UNAPPROVED.") update friends set status = ".USER_APPROVED."where requestId = ".$userId." and providerId in (select Id from users where username in (".$friendNamesQueryPart.") delete from friends where requestId = ".$userId." and providerId in (select Id from users where username in (".$friendNamesQueryPart.") select Id from users where username = '".$username."' and password = '".$password."' limit 1