Introduction
CHECK constraints with a simple condition
CHECK constraints with complex conditions
Adding CHECK constraints to existing tables
Instruction
Great! What about deleting a constraint? Let's go back to the video_game_ranking table:
CREATE TABLE video_game_ranking ( game_id integer, user_rating decimal(4, 1), expert_rating decimal(4, 1), rank integer, award_count integer CHECK (award_count > 0) );
Let's say we want to delete the constraint from the award_count column. Here's how we'd do it:
ALTER TABLE video_game_ranking DROP CONSTRAINT video_game_ranking_award_count_check;
Again, we're using code which should be familiar to you – you can delete primary/foreign keys and the UNIQUE constraints in the same way. The default naming convention for a CHECK constraint in PostgreSQL is table_name + _ + column_name(s) + _ + check. In our example, the check was made on the award_count column, so the constraint was named video_game_ranking_award_count_check.
Exercise
You are again given the train_connection table:
CREATE TABLE train_connection ( id integer PRIMARY KEY, city_from varchar(32), city_to varchar(32), distance_miles integer, basic_price decimal(5, 2) CHECK (basic_price > 0.0) );
It turns out that some train connections are completely free of charge. Your task is to remove the constraint on the basic_price column.
