Primary,composite and Foreign key in DBMS

Primary Key:

A table contains the data related entities. If you take STUDETNS table, it contains data related to students. For each student there will be one row in the table. Each student’s data in the table must be uniquely identified. In order to identify each entity uniquely in the table, we use a column in the table. That column, which is used to uniquely identify entities (students) in the table is called as primary key. In case of STUDENTS table we can use ROLL NO as the primary key as it in not duplicated. So a primary key can be defined as a set of columns used to uniquely identify rows of a table. Some other examples for primary keys are account number in bank, product code of products, employee number of an employee.

Composite Primary Key:

In some tables a single column cannot be used to uniquely identify entities (rows). In that case we have to use two or more columns to uniquely identify rows of the table. When a primary key contains two or more columns it is called as composite primarykey.  we have PAYMENTS table, which contains the details of payments made by the students. Each row in the table contains roll number of the student, payment date and amount paid. Neither of the columns can uniquely identify rows. So we have to combine ROLLNO and DP to uniquely identify rows in the table. As primary key is consisting of two columns it is called as composite primary key.

Foreign Key:

In relational model, we often store data in different tables and put them together to get complete information. For example, in PAYMENTS table we have only ROLLNO of the student. To get remaining information about the student we have to use STUDETNS table. Roll number in PAYMENTS table can be used to obtain remaining information about the student. The relationship between entities student and payment is one-to-many. One student may make payment for many times. As we already have ROLLNO column in PAYMENTS table, it is possible to join with STUDENTS table and get information about parent entity (student). Roll number column of PAYMENTS table is called as foreign key as it is used to join PAYMENTS table with STUDENTS table. So foreign key is the key on the many side of the relationship. ROLLNO column of PAYMENTS table must derive its values from ROLLNO column of STUDENTS table. When a child table contains a row that doesn’t refer to a corresponding parent key, it is called as orphan record. We must not have orphan records, as they are result of lack of data integrity.