SQL PRIMARY KEY

A primary key is used to uniquely identify each row on a table. It can either be part of the actual record itself, or it can be an artificial field (something that has nothing to do with the actual record). A primary key can consist of one or more fields on a table. When multiple fields are used as a primary key, it is called a composite key.

Primary keys can be specified either when the table is created (using CREATE TABLE) or by changing the existing table structure (using ALTER TABLE).

Below is an example of specifying a primary key when creating a table:

MySQL:

 

Oracle:

 

SQL Server:

 

 

Below are examples for specifying a primary key by altering a table:

MySQL:

 

 

Oracle:

 

 

SQL Server:

 

 

Note: Before using the ALTER TABLE command to add a primary key, you’ll need to make sure that the field is defined as ‘NOT NULL‘ — in other words, NULL cannot be an accepted value for that field.