Select distinct values from db

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
        )

Answer 2

I suggest using an anti-join pattern. But the specification isn't very clear. (Sample data and expected output would go a long ways towards clarifying it.)

SELECT u.id
     , u.username
  FROM user u
  LEFT
  JOIN app a
    ON a.userid = u.id
   AND a.mainid <=> ?
   AND a.appid  <=> ?
 WHERE a.userid IS NULL

This query will return rows from u where there is not a matching row in a that has the specified values for mainid and appid.

Given contents of user table as

  id  username
----  --------
   2  foo
   3  fee
   5  fi

and app table as

userid   appid   mainid 
------   -----   ------
     2     444      888
     3     444        1
     3       1      888

if we specify a value of 444 for appid and 888 for mainid in the query, eg

SELECT u.id
     , u.username
  FROM user u
  LEFT
  JOIN app a
    ON a.userid = u.id
   AND a.mainid <=> 444
   AND a.appid  <=> 888
 WHERE a.userid IS NULL

this query would return users 3 and 5. (User 2 will not be returned, because there is a row in app that matches the specification.)

---

There are other query patterns that will return an equivalent result. Using a NOT EXISTS (correlated subquery) pattern is probably the easiest pattern to understand.