MySQL - Are “NOT NULL” constraints needed for primary keys?

Question

is it necessary to declare "NOT NULL" constraints for primary keys in the MySQL database? A primary key cannot have NULL values because it auto_increments anyway and automatically fills the field record. So am I correct in saying this mean I can remove the "NOT NULL" constraint for my primary keys?

Answer 1

(As you've tagged your question mysql .) In MySQL, you don't have to do it explicitly. From the manual :

A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL . If they are not explicitly declared as NOT NULL , MySQL declares them so implicitly (and silently).

Of course, just because you don't have to doesn't mean you might not want to for clarity, etc.

Answer 2

Yes and no You can remove "Not null", that won't remove the constraint though. Personally I'd leave them in, you gain nothing worthwhile from taking them out.

Answer 3

Primary key must not include nullable columns. auto_increment is not a check constraint, (it is rather a default constraint) , so you cannot remove not null from definition of the column that is part of primary key regardless of presence of auto_increment . You don't have to type not null while creating the table for primary key in mysql, because the engine adds this constraint automatically.

Answer 4

WE need not declare explicitly the column as not null because Primary key constraint makes the column NOT NULL. I have checked in Oracle.