SQL联接和子查询，SQL联接的子查询

Question

I wrote this SQL query to get friends of user who is not in the invitation table , and it works will , however i read that subqueries are performance eaters and I am not very good with JOINS, any help for modifying this query to be just in joins will be really appreciated.

Here is the SQL Query

SELECT friend.id,
       friend.first_name
FROM   friends AS friend
       INNER JOIN friends_users AS friendsUser
         ON ( friend.id = friendsUser.friend_id
              AND friend.id NOT IN (SELECT friend_id
                                    FROM   friends_invitations
                                    WHERE  friends_invitations.user_id = 1) )
ORDER  BY friendsUser.id ASC 

Here is the tables structure

friends
id   first_name

friends_users
id  friend_id  user_id

friends_invitations
id  friend_id user_id

Any help will be really appreciated

Answer 1

A subquery is not necessarily a performance eater, is your query presenting performance issues?. Anyway, most times the fastest way to do your query would be using a NOT EXISTS :

SELECT  friend.id, 
        friend.first_name 
FROM friends AS friend 
INNER JOIN friends_users AS friendsUser 
    ON friend.id = friendsUser.friend_id  
WHERE NOT EXISTS (SELECT 1 FROM friends_invitations 
                  WHERE friends_invitations.user_id = 1
                  AND friend_id = friend.id)
ORDER BY friendsUser.id ASC

Here is a link explaining the differences between NOT IN , LEFT JOIN and NOT EXISTS (for SQL Server). So, test this different options and choose the right one for your tables.

Answer 2

you can use LEFT JOIN for this,

SELECT  friend.id,
        friend.first_name
FROM    friends AS friend
        INNER JOIN friends_users AS friendsUser
            ON friend.id = friendsUser.friend_id 
        LEFT JOIN   friends_invitations
            ON friend.id = friends_invitations.friend_id AND
                friends_invitations.user_id = 1
WHERE   friends_invitations.friend_id IS NULL
ORDER   BY friendsUser.id ASC