With a table created with this command:
CREATE TABLE people ( age INT NOT NULL, name CHAR(20) NOT NULL );
We can insert an item more than once.
And in particular, we can have columns that repeat the same value.
We can force a column to have only unique values using the
UNIQUE key constraint:
CREATE TABLE people ( age INT NOT NULL, name CHAR(20) NOT NULL UNIQUE );
Now if you try to add the ‘Flavio’ twice:
INSERT INTO people VALUES (37, 'Flavio'); INSERT INTO people VALUES (20, 'Flavio');
You’d get an error:
ERROR: duplicate key value violates unique constraint "people_name_key" DETAIL: Key (name)=(Flavio) already exists.
A primary key is a unique key that has another property: it’s the primary way we identify a row in the table.
CREATE TABLE people ( age INT NOT NULL, name CHAR(20) NOT NULL PRIMARY KEY );
The primary key can be an email in a list of users, for example.
The primary key can be a unique
id that we assign to each record automatically.
Whatever that value is, we know we can use it to reference a row in the table.
More database tutorials:
- Data models
- The Relational Model
- PostgreSQL User Permissions
- How MongoDB is different from a SQL database
- How to install SQLite on macOS
- How to install PostgreSQL on macOS
- How to list tables in the current database using PostgreSQL
- Relational Databases
- PostgreSQL vs MySQL, a comparison