Question

I have two mysql tables – a sales table:

+----------------+------------------------------+------+-----+---------+-------+
| Field          | Type                         | Null | Key | Default | Extra |
+----------------+------------------------------+------+-----+---------+-------+
| StoreId        | bigint(20) unsigned          | NO   | PRI | NULL    |       |
| ItemId         | bigint(20) unsigned          | NO   |     | NULL    |       |
| SaleWeek       | int(10) unsigned             | NO   | PRI | NULL    |       |
+----------------+------------------------------+------+-----+---------+-------+

and an items table:

+--------------------+------------------------------+------+-----+---------+-------+
| Field              | Type                         | Null | Key | Default | Extra |
+--------------------+------------------------------+------+-----+---------+-------+
| ItemId             | bigint(20) unsigned          | NO   | PRI | NULL    |       |
| ItemName           | varchar(100)                 | NO   |     | NULL    |       |
+--------------------+------------------------------+------+-----+---------+-------+

The sales table contains multiple records for each ItemID – one for each SaleWeek. I want to select all items sold by joining the two tables like so:

SELECT items.ItemName, items.ItemId FROM items
JOIN sales ON items.ItemId = sales.ItemId 
WHERE sales.StoreID = ? ORDER BY sales.SaleWeek DESC;

However, this is returning multiple ItemId values based on the multiple entries for each SaleWeek. Can I do a distinct select to only return one ItemID – I don’t want to have to query for the latest SaleWeek because some items may not have an entry for the latest SaleWeek so I need to get the last sale. Do I need to specify DISTINCT or use a LEFT OUTER JOIN or something?

Answer 1

A DISTINCT should do what you’re looking for:

SELECT DISTINCT items.ItemName, items.ItemId FROM items
JOIN sales ON items.ItemId = sales.ItemId 
WHERE sales.StoreID = ? ORDER BY sales.SaleWeek DESC;

That would return only distinct items.ItemName, items.ItemId tuples.