What's the impact of NULL on MySQL tables? (InnoDB)

Question

Hello guys,

Recently, I've been thinking if it's worth to have 1 table with perhaps a lot of NULL columns or if it's better to have more tables with no NULLs. I've heard that NULL isn't stored on InnoDB, so I was wondering if there is any downside or problem with having a bunch of rows with a lot of NULLs. I have always heard that common NULLs are bad, but I have never really learned why. By the way, those will be NULLs on foreign keys, if that matters at all.

On a second doubt, is there any performance issue when I'm using INNER JOIN on columns that have a lot of NULL? Like, if I have 4 foreign keys, and I'm going to do 4 INNER JOINs, but most likely only 1 of them is not NULL, is this going to affect perfomance? Thanks

Answer 1

See:

• Does mysql index null values?
• NULL in MySQL (Performance & Storage)

NULLs are indexed.

In InnoDB, you can reduce the storage requirements for your data row by using NULL.