Introduction
Defining columns with UNIQUE
Adding and removing UNIQUE constraints
Instruction
Very well done! We know how to add UNIQUE constraints when we define a table. How do we add a UNIQUE constraint to an existing table? Let's say we have the following table:
CREATE TABLE video_game ( id integer PRIMARY KEY, name varchar(64), platform varchar(64), price decimal(6, 2) );
We'd like to add a UNIQUE constraint to a group of two columns: name and platform. Take a look:
ALTER TABLE video_game ADD UNIQUE (name, platform);
The query above will add a UNIQUE constraint to our table. However, keep in mind that all existing name-platform combinations must be unique. If that's not the case, we won't be able to add the constraint. What's more, once we do add it, each new name-platform combination must be unique as well.
Exercise
We've got a table named card_game that's defined like this:
CREATE TABLE card_game ( id integer PRIMARY KEY, name varchar(64) UNIQUE, rank integer );
Your task is to add a UNIQUE constraint to the rank column.
