Primary keys

Introduction to Relational Databases in SQL

Timo Grossenbacher

Data Journalist

Primary keys

One primary key per database table, chosen from candidate keys
Uniquely identifies records, e.g. for referencing in other tables
Unique and not-null constraints both apply
Primary keys are time-invariant: choose columns wisely!

Specifying primary keys

CREATE TABLE products (
    product_no integer UNIQUE NOT NULL,
    name text,
    price numeric
);
CREATE TABLE products (
    product_no integer PRIMARY KEY,
    name text,
    price numeric
);

Taken from the PostgreSQL documentation.

CREATE TABLE example (
    a integer,
    b integer,
    c integer,
    PRIMARY KEY (a, c)
);

Specifying primary keys (contd.)

ALTER TABLE table_name
ADD CONSTRAINT some_name PRIMARY KEY (column_name)

Let's practice!

Introduction to Relational Databases in SQL