Introduction to Database Constraints

What are Constraints?

Database constraints are rules that limit the type of data that can be inserted into a table. These rules ensure data integrity and help maintain the accuracy and reliability of the data within the database.

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**: A combination of NOT NULL and UNIQUE. It uniquely identifies each row in a table.
• **FOREIGN KEY**: Ensures referential integrity by linking two tables together.
• **CHECK**: Ensures that all values in a column satisfy a specific condition.
• **DEFAULT**: Provides a default value for a column when no value is specified.

Implementing Constraints

Constraints can be defined when creating a table or can be added to an existing table using ALTER TABLE statements. Below are examples of how to implement various constraints using SQL.

-- Create a table with constraints
CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    FirstName VARCHAR(100) NOT NULL,
    LastName VARCHAR(100) NOT NULL,
    Email VARCHAR(255) UNIQUE,
    HireDate DATE NOT NULL,
    Salary DECIMAL(10, 2) CHECK (Salary > 0)
);

-- Adding a FOREIGN KEY constraint to an existing table
ALTER TABLE Orders
ADD CONSTRAINT fk_customer
FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID);

Best Practices

To effectively use database constraints, consider the following best practices:

1. Define constraints during the table creation process to ensure data integrity from the start.
2. Use primary keys to uniquely identify records and establish relationships between tables.
3. Regularly review and update constraints as your application and data model evolve.
4. Test constraints with various data inputs to ensure they behave as expected.

FAQ