sql - Mysql query to join three tables

Question

I am using this query:

SELECT a.sales_id, d.bus_title, a.cat_id
FROM tbl_sales a
INNER JOIN tb_category b ON a.cat_id = b.cat_id
INNER JOIN tbl_business d ON d.bus_id = a.bus_id

which produces this result:

sales_id  | bus_title      |cat_id
----------|----------------|------------
 1        | Business 1     | 6  
 2        | Business 12    | 12
 3        | Business 123   | 25

I changed the field cat_id into a new table named tb_sales_category which contains the fields sales_category_id, sales_id, cat_id. How can I write the new query by joining this table too to, get the same result as above?

I am kind of new to databases, need help. Thanks in advance

Answer 1

Try this:

SELECT a.sales_id, d.bus_title, s.cat_id
FROM tbl_sales a
INNER JOIN tb_sales_category s ON a.sales_id = s.sales_id
INNER JOIN tbl_business      d ON a.bus_id   = d.bus_id
INNER JOIN tb_category       b ON s.cat_id   = b.cat_id

The idea is fairly simple, the first field in your new table tb_sales_category which is sales_category_id is working as a surrogate key, it has nothing to do with the relations between the two other tables. Then we come to the other two fields which are sales_id, cat_id, these what you should map to the other two sides of the relations.

You can't Join tb_category b ON a.cat_id = b.cat_id on the new schema becouse we no longer have a.cat_id, and here comes the new table tb_sales_category role, by inserting it with two binding sides, one with INNER JOIN tb_category b ON s.cat_id = b.cat_id and the other with INNER JOIN tb_sales_category s ON a.sales_id = s.sales_id we should be done.

Hope this makes sense.