Constraints

CREATE TABLE employees (
    id INT PRIMARY KEY,                     
    first_name VARCHAR(50) NOT NULL,       
    last_name VARCHAR(50) NOT NULL,         
    email VARCHAR(100) UNIQUE,             
    birth_date DATE CHECK (birth_date > '1900-01-01'),  
    hire_date DATE DEFAULT CURRENT_DATE,   
    department_id INT,
    FOREIGN KEY (department_id) REFERENCES departments(id)
);

When creating tables, you can apply various constraints to the columns to enforce rules.

• PRIMARY KEY: Uniquely identifies each record in a table.
• FOREIGN KEY: Uniquely identifies a record in another table.
• NOT NULL: Ensures that a column cannot have a NULL value.
• UNIQUE: Ensures that all values in a column are different.
• CHECK: Ensures that all values in a column satisfy a specific condition.
• DEFAULT: Sets a default value for a column when no value is specified.