Primary Key in DBMS

A primary key has the following properties

• A primary key column must contain unique values
• Null values not allowed for the primary key column
• Only one primary key is allowed for a table.

Examples for primary keys 

• Student ID in student table is a primary key because no two students will have the same Id
• Employee ID in employee table is a primary key because no two employees to have the same Id.
• Age, name, phone numbers will not make sense  to be used as primary key columns because more than one person can have same age name our phone numbers, etc.

Creating a primary key 

A particular column is made as a primary key column by  using the primary key keyword followed with the column name

CREATE TABLE EMP
(
   ID   INT              
   NAME VARCHAR (20)     
   AGE  INT             
   ADDRESS  CHAR (25) ,
   SALARY   DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

• Here we have used the primary key on ID column then ID column must contain unique values i.e one ID cannot be used for another student.
• If you try to enter  duplicate value while inserting in the  row you are displayed with an error
• Hence primary key will restrict you to maintain unique values and not null values in that particular column.

Students Table

Primary key with more than one attribute 

• Consider a table  with three  attitudes customer ID, product ID, product quantity
• Customer ID needs to be entered for each time the customer purchases an  order hence customer ID appears more than once in the customer ID table hence it cannot be served as the primary key i.e it failed to uniquely identify a record
• Example customer ID 66 has placed two orders hence customer ID appeared 66 two times in the customer ID column

• Product  ID and product quality cannot be declared as the primary key because of more than one customer purchase same product and the same quantity
• In this situation all three attributes fail  to serve as a primary key .Hence  combination of these  attribute can be used as a primary key
• For example [customer ID, product ID] can be used as the primary key table customers this combination helps to uniquely access records of customer

Example 

Create table ORDER
(
    Customer_ID int ,
    Product_ID int ,
    Order_Quantity int ,
    Primary key (Customer_ID, Product_ID)
)

• While choosing a set of attributes for a primary key, we always choose the minimal set that has a minimum number of attributes.
• For example, if there are two sets that can identify a row in the table, the set that has a minimum number of attributes should be chosen as the primary key.