How do I run this MySQL JOIN query? - mysql

Question

Let's say I have 2 tables.
The first table is a list of personas. A user can have many personas.
mysql> select id, user_id, name from personas_personas;
+----+---------+--------------+
| id | user_id | name |
+----+---------+--------------+
| 8 | 1 | startup |
| 9 | 1 | nerd |
| 10 | 1 | close |
| 12 | 2 | Nerd |
| 13 | 2 | Startup |
| 14 | 2 | Photographer |
+----+---------+--------------+
6 rows in set (0.00 sec)
Now, I have another table called "approvals".
mysql> select id, from_user_id, to_user_id, persona_id from friends_approvals;
+----+--------------+------------+------------+
| id | from_user_id | to_user_id | persona_id |
+----+--------------+------------+------------+
| 2 | 1 | 2 | 8 |
| 3 | 1 | 2 | 9 |
+----+--------------+------------+------------+
2 rows in set (0.00 sec)
If from_user wants to approve to_user to a persona, then a record is inserted.
I'm trying to do this query...
Given a user, find all its personas. Then, for each persona, determine if it's approved for a certain to_user. If so, return is_approved=1 in the result set. Otherwise, return is_approved=0 in the result set.
So this is where I start:
SELECT *
FROM personas_personas
WHERE user_id = 1
LEFT JOIN friends_approvals ON
...but i don't know where to go from here.
So, the final result set should have all the columns in the personas_personas table, and then also is_approved for each of the results.

Answer 1

SELECT
pp.*,
CASE
WHEN exists (
SELECT
*
FROM
friends_approvals fa
WHERE
fa.from_user_id = pp.user_id AND
fa.persona_id = pp.id AND
fa.to_user_id = 2
)
THEN 1
ELSE 0
END as is_approved
FROM
personas_personas pp
WHERE
pp.user_id=1
Or, depending on your taste:
SELECT
pp.*,
CASE
WHEN fa.from_user_id IS NOT NULL
THEN 1
ELSE 0
END as is_approved
FROM
personas_personas pp
LEFT OUTER JOIN friends_approvals fa ON
pp.user_id = fa.from_user_id AND
pp.id = fa.persona_id AND
fa.to_user_id = 2
WHERE
pp.user_id=1

Answer 2

If I'm understanding your needs correctly, you can do this:
SELECT personas_personas.*,
CASE WHEN friends_approvals IS NULL THEN 0 ELSE 1 END AS is_approved
FROM personas_personas
LEFT
OUTER
JOIN friends_approvals
ON friends_approvals.from_user_id = ...
AND friends_approvals.to_user_id = personas_personas.user_id
AND friends_approvals.persona_id = personas_personas.id
WHERE personas_personas.user_id = ...
;
That will find every personas_personas record with the specified user_id, together with an indicator of whether that user, in that persona, has been "approved" by a specified from_user_id.
(If that's not what you want, then please clarify!)