Question

I need to query the database by joining two tables. Here is what I have:

Table Town:

• id
• name
• region

Table Supplier:

• id
• name
• town_id

I currently have the following query which outputs all the Towns that belong to a given region:

SELECT id, name FROM Town WHERE region = 'North West';

Now I need to extend this query and create two further queries as follows:

• Output the number of Suppliers for each Town

• Output only the Towns that have 1 or more Supplier

I am using PHP for my scripts if that helps. I know I may be able to to get this data using PHP but in terms of performance it will probably be better if it is done in MySQL.

EDIT (27/07/10):

I now needs to extend this one last time – there is another table called Supplier_vehicles:

• id
• supplier_id
• vehicle_id

A Supplier can have many Supplier_vehicles. The count (NumSupplier in this case) needs to now contain the total number of suppliers in a given town that have any of the given vehicle_id (IN condition):

SELECT * FROM Supplier s, Supplier_vehicles v WHERE s.id = v.supplier_id AND v.vehicle_id IN (1, 4, 6)

Need to integrate the above query into the existing JOIN query.

Answer 1

Count the number of suppliers.

SELECT t.id, t.name, count(s.id) as NumSupplier
FROM Town t
LEFT OUTER JOIN Suppliers s ON s.town_id = t.id
GROUP BY t.id, t.name

Only towns that have at least one supplier

SELECT DISTINCT t.id, t.name
FROM Town t
INNER JOIN Suppliers s ON s.town_id = t.id

And you are 100% correct, the best place for this is an SQL query.