Question

Lets say I have the following tables:

TABLE: topic
COLUMNs: topicId

TABLE: topicPermissions
COLUMNs: topicId, teamId

I want to write a query that pulls 10 unique topic id’s and all of their associated permissions rows. Each topic id can have multiple topicPermission rows assigned to it, but I only want 10 distinct topic id’s.

SELECT topic.topicId, topicPermission.teamId FROM topic LEFT JOIN topicPermissions ON topicId LIMIT 10

Obviously this doesn’t work because it limits it to 10 rows. Any help appreciated.

Just to clarify further: Another way to accomplish this would be to keep pulling rows from topic permissions until topic permissions has 10 distinct topic id’s. Is that possible?

Thanks.

Answer 1

This might work:

SELECT `TP`.`topicId`, `PM`.`teamId`
FROM (SELECT `topicId`
    FROM `topic`
    LIMIT 10
) `TP`
LEFT JOIN `topicPermissions` `PM` ON `TP`.`topicId` = `PM`.`topicId`;

Hope it helps!

EDIT 1:
Here is a test run for the above query.

Table “topic” has 13 records:

mysql> select * from topic;
+---------+
| topicId |
+---------+
|       1 |
|       2 |
|       3 |
|       4 |
|       5 |
|       6 |
|       7 |
|       8 |
|       9 |
|      10 |
|      11 |
|      12 |
|      13 |
+---------+
13 rows in set (0.00 sec)

Table “topicPermissions” has 4 entries for topicId = 1 and 2 entries for topicId = 5:

mysql> select * from topicPermissions;
+---------+--------+
| topicId | teamId |
+---------+--------+
|       1 |      1 |
|       1 |      2 |
|       1 |      3 |
|       1 |      4 |
|       5 |      1 |
|       5 |      9 |
+---------+--------+
6 rows in set (0.00 sec)

The resultant:

mysql> SELECT `TP`.`topicId`, `PM`.`teamId`
    -> FROM (SELECT `topicId`
    ->     FROM `topic`
    ->     LIMIT 10
    -> ) `TP`
    -> LEFT JOIN `topicPermissions` `PM` ON `TP`.`topicId` = `PM`.`topicId`;
+---------+--------+
| topicId | teamId |
+---------+--------+
|       1 |      1 |
|       1 |      2 |
|       1 |      3 |
|       1 |      4 |
|       2 |   NULL |
|       3 |   NULL |
|       4 |   NULL |
|       5 |      1 |
|       5 |      9 |
|       6 |   NULL |
|       7 |   NULL |
|       8 |   NULL |
|       9 |   NULL |
|      10 |   NULL |
+---------+--------+
14 rows in set (0.00 sec)

As I understand from your question, the query gives you 10 distinct topic ids and all topicPermissions for each topic id. No?

Please note that if you need to pick specific topic ids, you may change the derived table query below to read the desired topics:

SELECT `topicId` FROM `topic` LIMIT 10;