SQL: Find Items in Table A Not in Table B
Goal: Find records from Table A (Students) that do not exist in Table B (Rooms)
Prerequisites: 2 Tables with relational data, Ability to run SQL Queries
There is often times you wish to find items in one table or query that are not in another table or query. This is commonly referred to amongst the sql community as a problem referred to as "A not in B". In this example I want to find any Student who does not have a room assigned to them. Some other examples that you might think of may be, finding invoices that don't have any payments, or perhaps find customers who don't have any orders. Regardless of the specifics, it is generally very simple to find records in one table who do not have any associated record in another table.
Here are my 2 tables:
uid firstName lastName --- --------- -------- 1 Joe Blow 2 Nancy Drew 3 Johnny Appleseed
roomid uid building ------ --- -------- 1 1 Monroe 2 3 Washington
Now in order to find student names who do not have a room on campus we can use the query below:
SELECT firstName, lastName FROM students LEFT JOIN rooms ON students.uid=rooms.uid WHERE rooms.uid IS NULL
which would return:
firstName lastName --------- --------- Nancy Drew
This is a great query to have handy as these types of situations are extremely common and has a high use case for application developers and reports.