Question

I have two databases: one contains the users:

persons

id
name
course

the other one contains the

voters

   id
   personID
   type

An entry to the voters table gets added when the user voted (type describes different topics a user can vote on)

I want to get all users for example where persons.course = '1' AND voters.type = '2' and have the output contain the following columns: id, name, hasVoted (where hasVoted returns true if the personID can be found in the voters table)

Answer 1

A left join may be what your looking for.

SELECT persons.id, persons.name, voters.personID IS NOT NULL AS hasVoted
FROM persons LEFT JOIN voters ON (persons.id = voters.personID)
WHERE persons.course = '1' AND voters.type = '2'
GROUP BY persons.id

http://dev.mysql.com/doc/refman/5.0/en/join.html

The left join will include all the matching rows from the left side of the join, and any matching pieces from the right side of the join. Any rows from the left side without a matching right side will be have the right side columns values of null.

As for the ‘IS NOT NULL’ or ‘IS NULL’ is used because ‘X = NULL’ doesn’t behave as it appears.

> SELECT 'A' = NULL, 'A' IS NULL, NULL = NULL, NULL IS NULL;
+------------+-------------+-------------+--------------+
| 'A' = NULL | 'A' IS NULL | NULL = NULL | NULL IS NULL |
+------------+-------------+-------------+--------------+
|       NULL |           0 |        NULL |            1 |
+------------+-------------+-------------+--------------+

More details about working with nulls:
http://dev.mysql.com/doc/refman/5.0/en/working-with-null.html