Primary keys – the basics
Single-column primary keys
Multi-column primary keys
NOT NULL
Instruction
And... success! We did not specify that the columns were NOT NULL, so we could leave them without a value. In other words, if a column doesn't have the NOT NULL constraint, its value can be a NULL.
If you want, you can stress the fact that a certain column can be NULL by writing the keyword NULL after the column type:
CREATE TABLE rpg_game (
id int PRIMARY KEY,
name varchar(32) NULL,
genre varchar(32),
classes_no integer,
complexity integer,
min_players integer
);
The above code would work just as well without the NULL keyword. Its point is to draw the reader's attention to the fact that a column is NULLable.
Exercise
We've added the keyword NULL to every column in the rpg_game table except for the id column. Go ahead and try to add a few rows without some of the NULLable columns. Observe what happens.
