Time and again, support questions pop up on the Firebird mailing lists related to “strange things” happening with NULLs in Firebird SQL. The concept seems difficult to grasp – perhaps partly because of the name, which suggests a “nothing” that won’t do any harm if you add it to a number or stick it to the back of a string. In reality, performing such operations will render the entire expression NULL.
This article explores the behaviour of NULL in Firebird SQL, points out common pitfalls and shows you how to safely deal with expressions that contain NULL or may resolve to NULL.
If all you want is a quick reference to refresh your memory, hop straight to the summary (which is really brief indeed).
