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.