How to find missing data from one table that is a join of two other tables I have two tables, stock and location, with a third table that retains the level of stock at each location, stock_loc_info. 

Each stock item should have a row for each location. 

What SQL query would show that the row B1 is missing from stock_loc_info?

stock.stockcode
---------------
A
B

location.locno
--------------
1
2

stock_loc_info.fkstockcode  stock_loc_info.fklocno
--------------------------  ----------------------
A                           1
A                           2
B                           2
You need all combinations from the two tables, so a CROSS JOIN and then remove the ones in the 3rd table, using either NOT EXISTS or EXCEPT:

SELECT 
    s.stockcode, l.locno
FROM
    dbo.stock AS s
  CROSS JOIN
    dbo.location AS l

EXCEPT

SELECT 
    si.fkstockcode, si.fklocno
FROM 
    dbo.stock_loc_info AS si ;