Good. As you can see,
UNIQUE on multiple columns works exactly the way
PRIMARY KEY does.
There is, however, one major difference between the two. We can only have one primary key in a given table, but we can have as many
UNIQUE constraints as we want. Take a look at the example:
CREATE TABLE board_game (
id IDENTITY UNIQUE,
price decimal(5, 2),
UNIQUE (name, genre),
UNIQUE (name, studio)
We now have one row with the
UNIQUE constraint and two separate
UNIQUE constraints, each containing two columns.
Is there a difference between one
UNIQUE constraint with multiple columns and separate
UNIQUE constraints for each column? There is. In the former case, all the columns must form a unique group of values. In the latter, each value in each column must be unique independently.