Introduction
UNIQUE
NOT NULL
CHECK
DEFAULT
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 IDENTITY PRIMARY KEY,
name varchar(32) UNIQUE NULL,
genre varchar(32),
classes_no int,
complexity int,
min_players int
);
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 table rpg_game except for the id column. Take your time and try to add a few rows without some of the NULLable columns. Observe what happens.
