There are MAINLY 7 types of keys in a database:

• Candidate Key: The candidate key REPRESENTS a SET of properties that can uniquely identify a table. Each table may have multiple candidate keys. One key amongst all candidate keys can be chosen as a primary key. In the below example since studentId and firstName can be considered as a Candidate Key since they can uniquely identify every TUPLE.
• Super Key: The super key defines a set of attributes that can uniquely identify a tuple. Candidate key and primary key are subsets of the super key, in other words, the super key is their superset.

• Primary Key: The primary key defines a set of attributes that are USED to uniquely identify every tuple. In the below example studentId and firstName are candidate keys and any one of them can be chosen as a Primary Key. In the given example studentId is chosen as the primary key for the student table.
• Unique Key: The unique key is very similar to the primary key except that primary keys don’t allow NULL values in the column but unique keys allow them. So essentially unique keys are primary keys with NULL values.
• Alternate Key: All the candidate keys which are not chosen as primary keys are considered as alternate Keys. In the below example, firstname and lastname are alternate keys in the database.
• Foreign Key:  The foreign key defines an attribute that can only take the values present in one table common to the attribute present in another table. In the below example courseId from the Student table is a foreign key to the Course table, as both, the tables contain courseId as one of their attributes.
• Composite Key:  A composite key refers to a combination of two or more columns that can uniquely identify each tuple in a table. In the below example the studentId and firstname can be grouped to uniquely identify every tuple in the table.