Question

I have 2 tables in my database that look like so:

clients
+-------------+
| id   | sms  |
|------+------|
| 1    | 0    |
| 2    | 1    |
| 3    | 1    |
| 4    | 1    |
+------+------+

clients_lists_relationships
+----------------------+
| listid   | clientid  |
|----------+-----------|
| 1        | 1         |
| 1        | 2         |
| 2        | 1         |
| 3        | 1         |
+----------+-----------+

Now what I’m trying to do is get a list of clients who are in a bunch of lists. I do that like so:

SELECT c.id, 
       l.* 
FROM   clients AS c, 
       clients_lists_relationships AS l 
WHERE  c.id = l.clientid 
       AND c.sms = '1' 
       AND ( l.listid = '1' 
              OR l.listid = '2' );

This does give me a list of the clients that I need. But because a client can be in more than one list I get the same client more than once. How would I limit this to only one row for each client no matter how many lists they are in?

Answer 1

Use GROUP BY:

SELECT c.id, 
       l.listid
FROM clients c
INNER JOIN clients_lists_relationships l
ON c.id = l.clientid
WHERE  c.sms = 1
       AND l.listid IN (1,2)
GROUP BY c.id

Note that by doing this you lose information on which lists the client was a member of. This means that you should probably not select anything from client_lists_relationships as this information is either redundant (clientid) or incomplete (listid).