Is there a reason not to use <=> (null safe equals operator) in mysql instead of =?

Question

MySQL provides a nice operator <=> that works with comparisons that could contain a null such as null <=> null or null <=> 5 etc. giving back intuitive results as many programming languages. Whereas the normal equals operator always just returns null, which catches many new MySQL users such as myself awry.

Is there a reason MySQL has both and not JUST the functionality in <=> ? Who really needs an operator that is effectively undefined with built in language types?

Answer 1

Who really needs an operator that is effectively undefined with built in language types?

You asked for some real-world examples. Here's a spurious one. Let's say that you have a residential youth programme or similar, and one of the requirements is that the kids only share a room with someone of the same sex. You have a nullable M/F field in your database - nullable because your data feed is incomplete (you're still chasing down some of the data). Your room-matching code should definitely not match students where t1.Gender<=>t2.Gender, because it could end up matching two kids of unknown gender, who might be of opposite genders. Instead, you match where they're equal and not both null.

That's just one example. I admit that the behaviour of NULL and the = operator have caused a lot of confusion over the years, but ultimately the fault probably lies with the plethora of online MySQL tutorials that make no mention of how NULL interacts with operators, nor of the existence of the <=> operator.

Answer 2

The big difference between null in mySQL and in programming languages is that in mySQL, null means unknown value while in programming it means undefined value.

In mySQL, null does not equal null (unknown does not equal unknown). While in programming languages, null does equal null (undefined equals undefined).