Excellent QOTD and discussion. Special thanks to john.arnott for his explanatory code samples.

There's an error in your sql above. You're inserting twice into the same table (@myothertab) and not at all into @myanothertab.

Nice catch!

As usual, I learn more from the discussion than the original question.

If this had been the question:

SET @aa = 'abcde';SET @bb = ' ab';SET @cc = 'ab '

I...

Putting MAX(col2) in the result set helped me see what was going on. WHERE clause evaluated first giving one row result set. This one row satisfies HAVING clause.

select...

Your finger muscle memory wouldn't allow you to write null=null! FWIW, I think the error helps emphasize the point about being careful when dealing with nulls.