从db中选择不同的值

Question

I have two tables

user:

id,
username 

and

app:

userid,
appid,
mainid 

I want to display users that dont have a record in this table.I get from address mainId and appid.If I run this query

SelectQuery("select  User.UserName
from User
INNER JOIN App
ON User.Id = App.UserId
where App.Mainid!= ".$var_MainId." and App.AppId<>".$var_AppId);

It doesnt show the users that may have same userid and mainid but not same appid

Answer 1

You could use a not exists clause. For example, to find users who don't have an app entry with MainId 123 and AppId 456:

select  u.UserName
from    User u
where   not exists
        (
        select  *
        from    App a
        where   a.UserId = u.Id
                and a.MainId = 123
                and a.AppId = 456
        )