Setting Up Constraints in Database Design

1. Introduction

Constraints are essential rules that ensure data integrity and consistency in a database. They enforce conditions on data entries, preventing invalid data from being stored.

2. Types of Constraints

• **NOT NULL**: Ensures that a column cannot have a NULL value.
• **UNIQUE**: Ensures that all values in a column are different.
• **PRIMARY KEY**: Uniquely identifies each record in a table. A combination of NOT NULL and UNIQUE.
• **FOREIGN KEY**: Ensures referential integrity between two tables.
• **CHECK**: Ensures that all values in a column satisfy a specific condition.

3. Setting Constraints

Constraints can be set during table creation or altered later. Here's how to set constraints in SQL:

CREATE TABLE Employees (
    EmployeeID INT NOT NULL PRIMARY KEY,
    LastName VARCHAR(255) NOT NULL,
    FirstName VARCHAR(255),
    Email VARCHAR(255) UNIQUE,
    DepartmentID INT,
    FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID)
);

3.1 Adding Constraints to Existing Tables

To add constraints to existing tables, you can use the ALTER TABLE statement:

ALTER TABLE Employees
ADD CONSTRAINT chk_Age CHECK (Age >= 18);

4. Best Practices

• Define constraints at the table level for better clarity.
• Use meaningful names for constraints for easier maintenance.
• Regularly review constraints to ensure they meet evolving business requirements.
• Always test constraints in a development environment before applying to production.

5. FAQ