3. Constraints

Constraints enforce rules on the data in a database table to maintain integrity and accuracy.

Ensures that a column cannot store NULL values.
- Used for mandatory fields (e.g., Name, Email).

Example:

CREATE TABLE Users (
    UserID INT PRIMARY KEY,
    Name VARCHAR(100) NOT NULL,  -- Name cannot be NULL
    Email VARCHAR(100) NOT NULL
);

UserID (PK)	Name	Email
1	John	john@example.com
2	NULL	jane@example.com

Ensures that all values in a column are distinct.
- Can be used on multiple columns.
- Allows one NULL value per column.

Example:

CREATE TABLE Users (
    UserID INT PRIMARY KEY,
    Email VARCHAR(100) UNIQUE NOT NULL  -- Ensures each email is unique
);

Ensures values meet a specific condition.
- Used for data validation (e.g., age must be positive).

Example:

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    Age INT CHECK (Age >= 18)  -- Ensures age is 18 or older
);

EmployeeID (PK)	Age
1	25
2	17

Assigns a default value when no value is provided.
- Ensures consistent data.

Example:

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    Status VARCHAR(20) DEFAULT 'Pending'  -- Default order status
);

OrderID (PK)	Status
1	Shipped
2	Pending