Primary Key

What is a primary key?

• A primary key is a column (or a group of columns) that uniquely identifies each row in a table.
• Primary key values must be unique - no two rows can have the same primary key value.
• A primary key column cannot contain NULL values.
• A table can have only one primary key, which may consist of a single column or multiple columns. A multi-column primary key is called a composite key.

Defining a primary key

• Define the primary key in the CREATE TABLE statement, either at column level or table level.
• You can also add it later with ALTER TABLE, but the key columns must already be declared NOT NULL.
• DB2 automatically creates a unique index to enforce the primary key.

Example

• Single-column primary key:-
  
  CREATE TABLE CUSTOMERS (ID INT NOT NULL, NAME VARCHAR(20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR(25), SALARY DECIMAL(18,2), PRIMARY KEY (ID));
• Composite primary key:-
  
  CREATE TABLE CUSTOMERS (ID INT NOT NULL, NAME VARCHAR(20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR(25), SALARY DECIMAL(18,2), PRIMARY KEY (ID, NAME));
• Add a primary key to an existing table:-
  
  ALTER TABLE CUSTOMERS ADD PRIMARY KEY (ID);