Re: Where should I use outer joins?

[Russ]

Given the following tables:

mysql> desc poll;
+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(10) unsigned |      | PRI | 0       | auto_increment |
| poll  | varchar(50)      |      |     |         |                |
+-------+------------------+------+-----+---------+----------------+

and

mysql> desc poll_votes;
+---------+------------------+------+-----+---------+----------------+
| Field   | Type             | Null | Key | Default | Extra          |
+---------+------------------+------+-----+---------+----------------+
| id      | int(10) unsigned |      | PRI | 0       | auto_increment |
| poll_id | int(10) unsigned |      |     | 0       |                |
| address | varchar(50)      |      |     |         |                |
+---------+------------------+------+-----+---------+----------------+

with data like:

mysql> select * from poll;
+----+--------+
| id | poll   |
+----+--------+
|  1 | perl   |
|  2 | python |
|  3 | C      |
|  4 | C++    |
+----+--------+
mysql> select * from poll_votes;
+----+---------+----------+
| id | poll_id | address  |
+----+---------+----------+
|  1 |       1 | junk     |
|  2 |       2 | junk     |
|  3 |       2 | morejunk |
+----+---------+----------+

My first pass is this statement:

         select p.poll 
           from poll p 
left outer join poll_votes pv 
             on pv.poll_id=p.id and pv.address='junk' 
          where pv.poll_id is null;

It returns:

+------+
| poll |
+------+
| C    |
| C++  |
+------+

We do a left outer join (so we can see what we *didn't* get) on the address you want and have the 'where' clause give us what the join didn't find ('where pv.poll_id is null').
This should give you the polls for which this address has not voted.

Note: I assume you want to restrict votes to one vote per address per poll. You can easily accomplish this by removing id from poll_votes and making the primary key be both poll_id and address. This way, MySQL will enforce your policy for you.

Russ