连接两个表。 Select 一个表中的所有行并且仅匹配另一表中的值？

Question

I have the following tables:

Locations
+-------------+----------------+
| Location_ID | Location_Name  |
+-------------+----------------+
|           1 | Administration |
|           2 | Parking        |
|           3 | Warehouse      |
|           4 | Shipping       |
|           5 | Factory        |
|           6 | Office         |
|           7 | Processing     |
+-------------+----------------+

Item_Quantity
+---------+-------------+-------------------+
| Item_ID | Location_ID | Location_Quantity |
+---------+-------------+-------------------+
|       1 |           3 |                10 |
|       1 |           5 |                50 |
|       2 |           3 |                 7 |
+---------+-------------+-------------------+

I am trying to get a list of all Location_IDs and Location_Names with the Location_Quantity for a specified Item_ID.

The expected result for Item_ID = 1 would be this:

+-------------+----------------+-------------------+
| Location_ID | Location_Name  | Location_Quantity |
+-------------+----------------+-------------------+
|           1 | Administration |                 0 |
|           2 | Parking        |                 0 |
|           3 | Warehouse      |                10 |
|           4 | Shipping       |                 0 |
|           5 | Factory        |                50 |
|           6 | Office         |                 0 |
|           7 | Processing     |                 0 |
+-------------+----------------+-------------------+

The expected result for Item_ID = 2 would be this:

+-------------+----------------+-------------------+
| Location_ID | Location_Name  | Location_Quantity |
+-------------+----------------+-------------------+
|           1 | Administration |                 0 |
|           2 | Parking        |                 0 |
|           3 | Warehouse      |                 7 |
|           4 | Shipping       |                 0 |
|           5 | Factory        |                 0 |
|           6 | Office         |                 0 |
|           7 | Processing     |                 0 |
+-------------+----------------+-------------------+

I have tried the following queries:

SELECT l.Location_ID, l.Location_Name, iq.Location_Quantity
FROM Locations l
LEFT JOIN Item_Quantity iq ON l.Location_ID = iq.Location_ID
WHERE iq.Item_ID = @Item_ID

SELECT l.Location_ID, l.Location_Name, iq.Location_Quantity
FROM Item_Quantity iq
LEFT JOIN Locations l ON l.Location_ID = iq.Location_ID
WHERE iq.Item_ID = @Item_ID

SELECT l.Location_ID, l.Location_Name, Location_Quantity = iif(iq.Location_Quantity IS NOT NULL, iq.Location_Quantity, 0)
FROM Locations l
LEFT JOIN Item_Quantity iq ON l.Location_ID = iq.Location_ID
WHERE iq.Item_ID = @Item_ID

All queries return only the rows with entries in Item_Quantity.

This is what I am getting for Item_ID = 1 for any of the above queries:

+-------------+----------------+-------------------+
| Location_ID | Location_Name  | Location_Quantity |
+-------------+----------------+-------------------+
|           3 | Warehouse      |                10 |
|           5 | Factory        |                50 |
+-------------+----------------+-------------------+

I would have thought a Left Join on the Locations table would give me all of the rows from the specified columns, but I must be understanding something incorrectly?

Can anyone see what I am doing wrong here?

Answer 1

The condition needs to go in the ON clause. Otherwise, the WHERE clause turns the outer join into an inner join.

You also want to convert the NULL to a 0 , so use COALESCE() :

SELECT l.Location_ID, l.Location_Name, COALESCE(iq.Location_Quantity, 0) as Location_Quantity
FROM Locations l LEFT JOIN
     Item_Quantity iq
     ON l.Location_ID = iq.Location_ID AND iq.Item_ID = @Item_ID;