Sql: "int default 1 not null" - how it works?

Question

Hi,
I'm learning now from my knowledge book and here's a code inside sql database creating, which I can't understand.
"(...)
xyz1 char (20) not null, //Okay
xyz2 int default 0 not null, //Emm.. okay?
xyz3 int default 1 not null, //...what?
(...)"
I've read that "not null" is better practise than "null", because empty memory, but I can't understand idea of "default 0 not null" and a fortiori "default 1 not null", isn't it mutually exclusive? For what is "not null" after "default 0 or 1"?
Help in advance.

Answer 1

NULL means unknown data. It is not equal to zero.

NOT NULL constraint means, that you can't insert a row without giving a value to that column, because that column cannot contain NULL (unknown data).
But it can contain zero value.