Why isn't this JOIN working? - mysql

Question

Database Tables
ss_merchant
+----------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+----------------+
| pk_merchant_id | bigint(20) | NO | PRI | NULL | auto_increment |
| name | varchar(45) | YES | | NULL | |
| website | varchar(100) | YES | | NULL | |
+----------------+--------------+------+-----+---------+----------------+
ss_merchant_store
+----------------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+-------------+------+-----+---------+----------------+
| pk_merchant_store_id | bigint(20) | NO | PRI | NULL | auto_increment |
| fk_pk_merchant_id | bigint(20) | YES | | NULL | |
| street | varchar(20) | YES | | NULL | |
| city | varchar(20) | YES | | NULL | |
| postcode | varchar(8) | YES | | NULL | |
| telephone | varchar(15) | YES | | NULL | |
| email | varchar(45) | YES | | NULL | |
+----------------------+-------------+------+-----+---------+----------------+
ss_merchant_store_rating
+-----------------------------+------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------------------+------------+------+-----+---------+----------------+
| pk_merchant_store_rating_id | bigint(20) | NO | PRI | NULL | auto_increment |
| fk_pk_merchant_store_id | bigint(20) | NO | | NULL | |
| rating | int(1) | YES | | NULL | |
+-----------------------------+------------+------+-----+---------+----------------+
and my query:
SELECT *
FROM ss_merchant
JOIN ss_merchant_stores
ON ss_merchant.pk_merchant_id = ss_merchant_stores.fk_pk_merchant_id
JOIN ss_merchant_store_rating
ON ss_merchant_stores.pk_merchant_store_id = ss_merchant_store_rating.fk_pk_merchant_store_id

Answer 1

There isn't anything specifically wrong with your join but it does make the assumption that all three tables have at least one row for each merchant_id. If you want to allow for non-existant merchant_store_rating rows consider using a LEFT JOIN
If there is no matching row for the
right table in the ON or USING part in
a LEFT JOIN, a row with all columns
set to NULL is used for the right
table. You can use this fact to find
rows in a table that have no
counterpart in another table:
SELECT left_tbl.* FROM left_tbl LEFT JOIN right_tbl
ON left_tbl.id = right_tbl.id WHERE right_tbl.id IS NULL;
This example finds all rows in
left_tbl with an id value that is not
present in right_tbl (that is, all
rows in left_tbl with no corresponding
row in right_tbl). This assumes that
right_tbl.id is declared NOT NULL.