primary key constraint example in sql

Example 1: primary key sql

A primary key is a field in a table which uniquely identifies each row/record in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values.

A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key.

If a table has a primary key defined on any field(s), then you cannot have two records having the same value of that field(s).

Example 2: primary key in sql

Primary Key :
It is unique column in every table in a database
It can ONLY accept;
    - nonduplicate values
    - cannot be NULL