SELECT certain fields based on certain WHERE conditions

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))

Answer 2

You can create to separate queries and then UNION the result tables. In each query, add a field that always has the same value.

So something like this should work:

(SELECT id, 'Not Friends' As Status FROM t1 WHERE condition1)
UNION
(SELECT id, 'Unconfirmed' As Status FROM t1 WHERE condition2)

Just make sure the same number and name of fields exists in both queries.