What is Primary key?

A primary key is used to uniquely identify each row in a table. It can either be part of the actual record itself, or it can be an artificial field (one that has nothing to do with the actual record). A table can have only one primary key. 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.

Below are examples for specifying a primary key when creating a table:

MySQL:
CREATE TABLE Customer
(SID integer,
Last_Name varchar(30),
First_Name varchar(30),
PRIMARY KEY (SID));

Oracle:
CREATE TABLE Customer
(SID integer PRIMARY KEY,
Last_Name varchar(30),
First_Name varchar(30));

SQL Server:
CREATE TABLE Customer
(SID integer PRIMARY KEY,
Last_Name varchar(30),
First_Name varchar(30));

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

MySQL:
ALTER TABLE Customer ADD PRIMARY KEY (SID);
Oracle:
ALTER TABLE Customer ADD PRIMARY KEY (SID);
SQL Server:
ALTER TABLE Customer ADD PRIMARY KEY (SID);

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.