Primary keys - the basics
Single-column primary keys
Multicolumn primary keys
Auto-increment columns and sequences
Instruction
Well, the insertion failed because the database keeps track of what is being added. If it finds out that the combination which constitutes the primary key has already been used, your instruction will be rejected.
Exercise
Let's see if you can create a new table based on the description below:
Each customer in our store has certain favorite genres. This is why we need a table named preference with the following information: user_id (an integer), priority (an integer), genre (text up to 20 characters). The first two columns form the primary key.
We created a simple template for you. Fill in the gaps with the table name, column names and column types!
Stuck? Here's a hint!
Type
CREATE TABLE preference ( user_id int, priority int, genre varchar(20), PRIMARY KEY (user_id, priority) );
