What is Primary Key, Foreign Key, Candidate Key, Alternative Key, Composite Key

Different keys in SQL SERVER

Primary key: Primary key uniquely identify a record in the table. Primary Key can't accept null values. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index, We can have only one Primary key in a table.

Foreign key: Foreign key is a field in the table that is referenced to primary key in another table, Foreign key can accept multiple null value,

Foreign key do not automatically create an index, you can manually create an index on foreign key, We can have more than one foreign key in a table.

Candidate key: A column, or set of columns that can uniquely identify a row in a table.

Alternate key: Any candidate key that has not been selected as the primary key.

Composite key: A key that is composed of more than one column.