This is something that wasn’t apparent to me for quite a while and seems to trip me up every now and again.
Say you have two tables. ‘Employees’ and ‘Payroll’.
Not great tables, but they will get the point across.
If you wanted to select all employees who have never been paid, you would normally do a query like this:
What trips up this query is if there is a NULL username in Payroll, it will never return any results. In order to make it work, you have to select all the NON NULL usernames from Payroll.
This behavior seems to be consistent in MySQL 5 and SQL Server 2005, I haven’t tested in Oracle.