Different types of Keys in sql server.

Candidate Key

Candidate key is a key of a table which can be selected as a primary key of the table. A table can have multiple candidate keys, out of which one can be selected as a primary key.

Primary Key

Primary key is a candidate key of the table selected to identify each record uniquely in table. Primary key does not allow null value in the column and keeps unique values throughout the column. In SQL Server, by default primary key creates a clustered index on a heap tables (a table which does not have a clustered index is known as a heap table). We can also define a nonclustered primary key on a table by defining the type of index explicitly.
A table can have only one primary key and primary key can be defined in SQL Server using below SQL statements:

1. CRETE TABLE statement (at the time of table creation) – In this case, system defines the name of primary key
2. ALTER TABLE statement (using a primary key constraint) – User defines the name of the primary key

Unique Key

Unique key is similar to primary key and does not allow duplicate values in the column. It has below differences in comparison of primary key:

1. It allows one null value in the column.
2. By default, it creates a nonclustered index on heap tables.

Alternate Key

Alternate key is a candidate key, currently not selected as primary key of the table.

Composite Key

Composite key (also known as compound key or concatenated key) is a group of two or more columns that identifies each row of a table uniquely. Individual column of composite key might not able to uniquely identify the record. It can be a primary key or candidate key also.

Super Key

Super key is a set of columns on which all columns of the table are functionally dependent. It is a set of columns that uniquely identifies each row in a table. Super key may hold some additional columns which are not strictly required to uniquely identify each row. Primary key and candidate keys are minimal super keys or you can say subset of super keys.

Foreign Key

In a relationship between two tables, a primary key of one table is referred as a foreign key in another table. Foreign key can have duplicate values in it and can also keep null values if column is defined to accept nulls.