How to retrieve data from two MySQL tables with multiple associations - mysql

Question

I got these two tables where one table is having multiple foreign keys to the second table.
Table rankings
+----------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+--------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| search_text | varchar(255) | YES | MUL | NULL | |
| first_item_id | bigint(20) | YES | MUL | NULL | |
| second_item_id | bigint(20) | YES | MUL | NULL | |
| third_item_id | bigint(20) | YES | MUL | NULL | |
| forth_item_id | bigint(20) | YES | MUL | NULL | |
+----------------+--------------+------+-----+---------+----------------+
Table item
+---------------------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------------------+--------------+------+-----+---------+----------------+
| id | bigint(20) | NO | PRI | NULL | auto_increment |
| item_code | varchar(255) | YES | MUL | NULL | |
+----------------+--------------+------+-----+---------+---------------------------+
One ranking record may have multiple association to item table using first_item_id, second_item_id, third_item_id or forth_item_id fields. I want to retrieve ranking records with the corresponding item_code instead of the item.id. What would be the most efficient way to do this if i have a big number of data?
PS: There are 10 associations to the item.id as first_item_id ... tenth_item_id. Im using Rails ActiveRecord ORM. Any workaround with that also fine.
Sample data ranking
SELECT id,search_text,first_item_id as first,second_item_id as second,third_item_id as third,forth_item_id as forth from rankings limit 10;
+----+-------------+-------+--------+-------+-------+
| id | search_text | first | second | third | forth |
+----+-------------+-------+--------+-------+-------+
| 1 | test 1 | 1 | 2 | 3 | 4 |
| 2 | test 2 | 1 | 2 | 3 | 4 |
| 3 | test 3 | 1 | 2 | 3 | 4 |
| 4 | test 4 | 1 | 2 | 3 | 4 |
+----+-------------+-------+--------+-------+-------+
Sample item data
SELECT id,item_code from items limit 5;
+--------+------------+
| id | item_code |
+--------+------------+
| 1 | 125659 |
| 2 | 125660 |
| 3 | 125661 |
| 4 | 125662 |
+--------+------------+
Expected data
+----+-------------+-------+--------+-------+-------+
| id | search_text | first | second | third | forth |
+----+-------------+-------+--------+-------+-------+
| 1 | test 1 | 125659| 125660 | 125661| 125662|
| 2 | test 2 | 125659| 125660 | 125661| 125662|
| 3 | test 3 | 125659| 125660 | 125661| 125662|
| 4 | test 4 | 125659| 125660 | 125661| 125662|
+----+-------------+-------+--------+-------+-------+

Answer 1

Joining the table multiple times (even many, many times) should not be a problem, as you are joining on the primary key, i.e. you have an index that will be used.
select
r.id,
r.search_text,
i1.item_code as item_code_1,
i2.item_code as item_code_2,
i3.item_code as item_code_3,
i4.item_code as item_code_4
from rankings r
left join item i1 on i1.id = r.first_item_id
left join item i2 on i2.id = r.second_item_id
left join item i3 on i3.id = r.third_item_id
left join item i4 on i4.id = r.forth_item_id
order by r.id;
I am using outer joins here, because all your item columns are nullable.