Comments on: Selecting records from one table that dont have specific relationship in a second table

By: Amy

Amy — Sat, 05 Jun 2010 01:50:37 +0000

SELECT A.* FROM A
LEFT OUTER JOIN L
ON L.question_id = A.question_id
AND L.category_id = X
WHERE L.category_id IS NULL

By: Hazan Ilan

Hazan Ilan — Wed, 27 Jan 2010 20:53:26 +0000

The condition (L.question_id = A.question_id AND L.category_id = X) at the ON clause force the left join to match only the records that DO have the X category. In other words, the records without the X category will be the records with null values. All you need, in order to get only those with the null values (records that not in category X), is to restrict the query to retrieve only the records with null by putting L.category_id IS NULL at the WHERE clause.

By: gloria

gloria — Wed, 27 Jan 2010 14:45:23 +0000

SELECT A.* FROM A
LEFT OUTER JOIN L
ON L.question_id = A.question_id
AND L.category_id = X
WHERE L.category_id IS NULL

I don’t understand that when we are taking the condition as L.category_id is NULL then Why are we taking the joining condition as L.Category_id=X

By: Hazan Ilan

Hazan Ilan — Tue, 19 Jan 2010 08:15:46 +0000

This is a great query. Thanks

By: rudy

rudy — Tue, 19 Jan 2010 04:02:31 +0000

SELECT A.* FROM A
LEFT OUTER JOIN L
ON L.question_id = A.question_id
AND L.category_id = X
WHERE L.category_id IS NULL

By: Burak

Burak — Tue, 19 Jan 2010 00:41:59 +0000

If you just need the questions, and not every single category:

SELECT A.*, SUM(IF(L.category_id = X,1,0)) as count_x LEFT JOIN L USING(question_id) WHERE count_x = 0 GROUP BY question_id;