A key is an attribute of a table which helps to identify a row. There can be many different types of keys which are explained here.
Super Key or Candidate Key: It is such an attribute of a table that can uniquely identify a row in a table. Generally they contain unique values and can never contain NULL values. There can be more than one super key or candidate key in a table e.g. within a STUDENT table Roll and Mobile No. can both serve to uniquely identify a student.
Primary Key: It is one of the candidate keys that are chosen to be the identifying key for the entire table. E.g. although there are two candidate keys in the STUDENT table, the college would obviously use Roll as the primary key of the table.
Alternate Key: This is the candidate key which is not chosen as the primary key of the table. They are named so because although not the primary key, they can still identify a row.
Composite Key: Sometimes one key is not enough to uniquely identify a row. E.g. in a single class Roll is enough to find a student, but in the entire school, merely searching by the Roll is not enough, because there could be 10 classes in the school and each one of them may contain a certain roll no 5. To uniquely identify the student we have to say something like “class VII, roll no 5”. So, a combination of two or more attributes is combined to create a unique combination of values, such as Class + Roll.
Foreign Key: Sometimes we may have to work with an attribute that does not have a primary key of its own. To identify its rows, we have to use the primary attribute of a related table. Such a copy of another related table’s primary key is called foreign key.