Primary Key Constraints
- A primary key is used to uniquely identify each row in a table.
- primary key mus contain value
- primary key column cannot contain NULL values.
- A primary key can consist of one or more fields on a table. When multiple fields are used as a primary key, they are called a composite key.
- Primary keys can be specified during alter table or create table
MySQL :
CREATE TABLE articles
(cat_id integer,
title varchar(30),
description varchar(30),
PRIMARY KEY (cat_id));
(cat_id integer,
title varchar(30),
description varchar(30),
PRIMARY KEY (cat_id));
Oracle :
CREATE TABLE articles
(cat_id integer PRIMARY KEY,
title varchar(30),
description varchar(30));
(cat_id integer PRIMARY KEY,
title varchar(30),
description varchar(30));
SQL Server :
CREATE TABLE articles
(cat_id integer PRIMARY KEY,
title varchar(30),
description varchar(30));
(cat_id integer PRIMARY KEY,
title varchar(30),
description varchar(30));
Below are examples for specifying a primary key by altering a table:
MySQL/MSSQL/Oracle:
ALTER TABLE articles ADD PRIMARY KEY (cat_id);