A key is the data item that exclusively identifies a record. For example, account number, Product code, employee number, and customer number are used as key fields because they identify a record stored in the database
	A super key for an entity is a set of one or more attributes whose combined value uniquely identifies the entities in the entity set. For example, for an entity set Employees, the set of attributes (emp_name, address) can be considered to be a super key, if we assume that there are no two employees with the same name emp_name and same address
	The primary key of a relation can be said to be a minimal super key. The field or group of fields which form the unique identifier for a table is called the table’s primary key. The primary key should be chosen such that its attributes are never or very rarely changed. For instance, the address field of a person should not be part of the primary key, since it is likely to change. Emp_code, on the other hand, is guaranteed to never change, till he is in the organization.
	There is only one primary key in a table. But there can be multiple candidate keys. A candidate key is an attribute or set of attributes that uniquely identifies a record. These attributes or combination of attributes are called candidate keys. In such a case, one of the candidate key is chosen as a primary key. The remaining candidate keys are called Alternate keys.
	In many cases, as we design the database, we will have tables that will use more than one column as part of the primary key. These are called composite keys (or concatenated keys). In other words, when a record cannot be uniquely identified by a single field, in such cases a composite key is used. A composite key is a group of fields that uniquely identify a record.
	In a relation, column whose data values correspond to the values of a key column in another relation is called a Foreign key. In a relational database the foreign key of a relation would be the primary key of another relation
	A secondary key is an attribute or combination of attributes that may not be a candidate key but classifies the entity set on a particular characteristic. For example, the entity set EMPLOYEE having the attribute ‘Department’ which identifies by its values which means all instances of EMPLOYEE who belong to a given department. More than one employee may belong to a department, so the Department attribute is not a candidate key for the entity set EMPLOYEE since it cannot uniquely identify an individual employee. However, the ‘Department’ attribute does identify all employees belonging to a given department.