Entity Integrity Rule in RDBMS

The Entity Integrity Rule in RDBMS states that every table must have a primary key, and the primary key column(s) cannot contain NULL values. This ensures that every row in a table is uniquely identifiable.

Example 1: Student Table

Consider the following Student table ?

Here Student_ID is the primary key. We cannot use Student_Awards as the primary key since not every student would have received an award (it can be NULL).

Example 2: Employee Table

The primary key is Employee_ID. It must have a unique, non-NULL value for every row.

Entity Integrity Rule Summary

• Every table must have a primary key.
• The primary key must have unique values for each row.
• The primary key cannot be NULL − for example, Employee_ID cannot be null in the Employee table.
• Every entity (row) must be uniquely identifiable.

SQL Example

The following SQL demonstrates entity integrity using the NOT NULL and PRIMARY KEY constraints ?

CREATE TABLE Student (
    Student_ID VARCHAR(10) NOT NULL,
    Student_Name VARCHAR(50),
    Student_Awards VARCHAR(50),
    PRIMARY KEY (Student_ID)
);

-- Valid: Student_ID has a value
INSERT INTO Student VALUES ('S001', 'Alice', 'Gold Medal');

-- Invalid: Student_ID is NULL (violates entity integrity)
-- INSERT INTO Student VALUES (NULL, 'Bob', NULL);
-- Error: Column 'Student_ID' cannot be null

Conclusion

The Entity Integrity Rule ensures that every row in a table is uniquely identifiable by requiring a non-NULL, unique primary key. This is a fundamental constraint in RDBMS that prevents duplicate or unidentifiable records in a table.