Question

I’m using MS SQL Server and I have 2 tables.

Supply_list

sl_id(pk) supply_id(fk)*       transaction_id    Qty 
1             14               872670099         3 
2             15               872670100         5 
3             16               872670101         1 
4             16               872670105         4 <

• supply_id is a foreign key to the supply_id in amenity table

Supply

supply_id(pk)    no_of_units 
----------------------------
13               2 
14               3
15               6
16               10

The output should be supply_id then the no. of units available which is equal to No_of_units subtracted by the Qty.

output

id      units available 
-------------------------
13           2 
14           0 --> [1]
15           1
16           5 --> [2]

[1] Since based on the supply_list table supply_id 14 has 3 for its Qty

[2] There are two records that contains supply_id 16 so we have to add their qty which are 4 and 1 so we have 5. And that 5 would be subtracted from the no_of_units of supply_id 16 and we will get 5 for units available.

Answer 1

• You left outer JOIN the list table to the parent table
• Subtract the SUM of the list Qty values from the parent no_of_units value
• Use ISNULL in case there are no list rows

Something like

SELECT
   S.supply_id,
   S.no_of_units - ISNULL(SUM(SL.Qty), 0) AS [units available]
FROM
   supply S
   LEFT JOIN
   supply_list SL ON S.supply_id = SL.supply_id
GROUP BY
   S.supply_id, S.no_of_units

This makes the aggregate more obvious but is the same query

SELECT
   S.supply_id,
   S.no_of_units - ISNULL(SL.SumListQty, 0) AS [units available]
FROM
   supply S
   LEFT JOIN
   (
   SELECT supply_id, SUM(Qty) AS SumListQty
   FROM supply_list
   GROUP BY supply_id
   ) SL ON S.supply_id = SL.supply_id