Question

I am writing an advanced MySQL query that searches a database and retrieves user information. What I am wondering is can I select certain fields if WHERE condition 1 is met and select other fields if WHERE condition 2 is met?

Database: users

------------------------
| user_id | first_name |
------------------------
|    1    |    John    |
------------------------
|    2    |    Chris   |
------------------------
|    3    |     Sam    |
------------------------
|    4    |    Megan   |
------------------------

Database: friendship

--------------------------------------
| user_id_one | user_id_two | status |
--------------------------------------
|      2      |      4      |    0   |
--------------------------------------
|      4      |      1      |    1   |
--------------------------------------

Status 0 = Unconfirmed
Status 1 = Confirmed

OK, as you can see John & Megan are confirmed friends while Chris & Megan are friends but the relationship is unconfirmed.

The query I am trying to write is as follow: Megan(4) searches for new friends I want all of the users except for the ones she is a confirmed friend with to be returned. So, the results should return 2,3. But since a relationship with user_id 2 exists but is not confirmed, I want to also return the status since an entry in the friendship table does exist between the two. If a user exist but there is no connection in the relationship table it still returns that users information but returns status as a NULL or doesn’t return status at all since it doesn’t exist in that table.

I hope this makes since. Ask questions if you need to.

Answer 1

Why not use a left join or an if-not-exists?

SELECT users.* 
FROM (users LEFT JOIN friendships 
       ON status=1 AND (user_id_one=user_id OR user_id_two=user_id) )
WHERE 
     status IS NULL

or

SELECT users.* 
FROM users
WHERE 
NOT EXISTS (SELECT * 
            FROM friendships 
            WHERE status=1 
            AND (user_id_one=user_id 
                 OR user_id_two=user_id))