One of my favorite uses of set-data manipulation involves using NULL values to my advantage; from NULLIF to COALESCE, we’ll explore some creative use of null values. These tips & tricks aren’t just a way to convert a NULL to another value, they’re a multi-purpose, insanely powerful way to massage and combine data.
Level Set: What is a NULL?
In SQL a NULL value isn’t a value at all – it’s lack of value. It’s a value that is indicative of not having a value. Think of it as if you asked someone a question but they didn’t respond, their response was a NULL value. So, therefore you cannot compare someone’s non-response to someone else’s non-response, while they seem like the same answer they’re likely not even the same question!
SELECT NULL WHERE NULL = NULL;
SELECT NULL WHERE NULL IS NULL;
There are some exceptions to this rule (thanks, Microsoft) but, like most of my blog posts, I try to stick with ANSI standard rules. While some RDBMSs treat NULL differently and even have switches that can be set during runtime to alter how NULL logic works, we’re not going to go there. Just assume ANSI 99.