primary column in sql code example

Example 1: change primary key sql

ALTER TABLE <Table_Name>
DROP CONSTRAINT <constraint_name>

ALTER TABLE <Table_Name>
ADD CONSTRAINT <constraint_name> PRIMARY KEY (<Column1>,<Column2>)

Example 2: 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).