SQL get table1 names with a count of table2 and table3 - sql

Question

I have three tables, table1 is connected to table2 and table3, but table2 and table3 are not connected. I need an output count of table2 and table3 for each table1 row. I have to use joins and a group by table1.name
SELECT Tb_Product.Name, count(TB_Offers.Prod_ID) 'Number of Offers', count(Tb_Requests.Prod_ID) 'Number of Requests'
FROM Tb_Product LEFT OUTER JOIN
Tb_Requests ON Tb_Product.Prod_ID = Tb_Requests.Prod_ID LEFT OUTER JOIN
TB_Offers ON Tb_Product.Prod_ID = TB_Offers.Prod_ID
GROUP BY Tb_Product.Name
I need to combine these queries:
SELECT Tb_Product.[Name], count(TB_Offers.Prod_ID) 'Number of Offers'
FROM Tb_Product LEFT OUTER JOIN
TB_Offers ON Tb_Product.Prod_ID = TB_Offers.Prod_ID
GROUP BY Tb_Product.[Name]
SELECT Tb_Product.[Name], count(Tb_Requests.Prod_ID) 'Number of Requests'
FROM Tb_Product LEFT OUTER JOIN
Tb_Requests ON Tb_Product.Prod_ID = Tb_Requests.Prod_ID
GROUP BY Tb_Product.[Name]
Results:
Name Number of Offers
Airplane 6
Auto 5
Bike 3
Camera 0
Computer 12
Milk 4
Oil 4
Orange 6
Telephone 0
Truck 6
TV 4
Name Number of Requests
Airplane 1
Auto 5
Bike 0
Camera 2
Computer 6
Milk 4
Oil 5
Orange 6
Telephone 0
Truck 1
TV 5
My results for offers and requests are the same value. I am not sure what I am doing wrong with the joins. Do I need to somehow join product to request and separately join product to offers? This needs to be done in one query.
This is for a class. Explanation would also be appreciated.

Answer 1

The simplest way to do this is to count the distinct values of each column:
SELECT
Tb_Product.Name,
count(distinct TB_Offers.Prod_ID) 'Number of Offers',
count(distinct Tb_Requests.Prod_ID) 'Number of Requests'
FROM
Tb_Product
LEFT OUTER JOIN
Tb_Requests ON Tb_Product.Prod_ID = Tb_Requests.Prod_ID
LEFT OUTER JOIN
TB_Offers ON Tb_Product.Prod_ID = TB_Offers.Prod_ID
GROUP BY
Tb_Product.Name
This is necessary because of the way joins work consecutively to produce a rowset that is a combination of all the input relations. COUNT() normally performs a count of non-null values in a column.
You can also do something like this, which aggregates the counts from the child tables independently and then joins them to the base table:
SELECT
p.Name,
o.cnt as Offer_Count,
r.cnt as Request_Count
FROM
TB_Product p
LEFT OUTER JOIN
(SELECT Prod_ID, COUNT(1) cnt FROM TB_Offers GROUP BY Prod_ID) o
LEFT OUTER JOIN
(SELECT Prod_ID, COUNT(1) cnt FROM TB_Requests GROUP BY Prod_ID) r
More explanation...
Let's say you have two products:
Prod_ID
Name
1
Widget
2
Gizmo
And two offers, one for each product:
Offer_ID
Prod_ID
100
1
200
2
And two requests for each product:
Request_ID
Prod_ID
1001
1
1002
1
2001
2
2002
2
Now you join Product relation to Offer relation on Prod_ID, you get a result like this:
Prod_ID
Name
Offer_ID
Prod_ID
1
Widget
100
1
2
Gizmo
200
2
Now when you join that relation to Requests on Prod_ID, you get something like this:
Prod_ID
Name
Offer_ID
Prod_ID
Request_ID
Prod_ID
1
Widget
100
1
1001
1
1
Widget
100
1
1002
1
2
Gizmo
200
2
2001
2
2
Gizmo
200
2
2002
2
Now when you count any of these columns you get 4 because each column has 4 values.