• In relational database primary key is a specific choice of minimal set of attributes that uniquely specify a relation.
• Primary key is a special realational database table column key that uniquely identifies a table record.
• In a table if primary key needs to be created then we have to define a primary key constraint when you create or modify a table.
• When multiple coulmns are declared as primary key then it is called as Composite key.
• A table should not contain more than one primary key.

A primary key’s main features are:

• It must contain a unique value for each row of data. A table can have only one primary key constraint
• It cannot contain null values.
• Every row must have a primary key value.
• It enforces the entity integrity of data.
• Primary key always has a unique data.
• Primary key length cannot be more than 900 bytes.
• Primary key cannot have a null value.
• Duplicate values are not allowed in primary key.
• A primary key might use one or more fields already present in the table, or a unique field can be created to be the primary key.