I am assuming you are using MySQL, so you could use BOOLEAN: [dev.mysql.com...]

Its a synonym for TINYINT so you are not going to get a smaller type, and its a lot clearer, may benefit from boolean specific improvements in the future, may be better understood by libraries.

If you have multiple booleans in a row you can save a bit of space by using the BIT type but it would complicate your code.

You're right, it's MySQL. For some reason, when I try to enter MySQL code this site occassionally gives me a FORBIDDEN error, so I took out the word "MySQL" when I was trying to get it to post.

Why would BIT complicate the code? I was considering that, since it uses 1 bit instead of 8 bits like TINYINT. But I saw that when I changed the column to BIT, the default NULL shows <em>NULL</em>, so I suspect the NULL is an issue?

BIT uses a minimum of a byte, so the only way it would save storage is if you store a different bit in each element of a byte. SO 1) it will not have any saving over SMALLINT/BOOL unless you have more than once true/false a row and 2) you might end up with manipulation to separate values stored in one byte.

A trick that I knew worked on Postgres, but which I just found works on MySQL too, is to use blank or NULL CHAR(0): [dev.mysql.com...] Just one bit.