SQL Comments

Example Table

We will use the following table named employees for our examples:

        CREATE TABLE employees (
            employee_id INT PRIMARY KEY,
            employee_name VARCHAR(50),
            department VARCHAR(50),
            salary DECIMAL(10, 2)
        );

        INSERT INTO employees (employee_id, employee_name, department, salary) VALUES
        (1, 'John Doe', 'HR', 50000.00),
        (2, 'Jane Smith', 'Finance', 60000.00),
        (3, 'Sam Brown', 'IT', 70000.00),
        (4, 'Nancy White', 'HR', 55000.00),
        (5, 'Mike Green', 'Finance', 65000.00);
        

Using SQL Comments

SQL comments are used to explain sections of SQL statements or to prevent execution of SQL statements. Comments are ignored by the SQL engine.

Single-line Comments

Single-line comments start with two consecutive hyphens (--). Any text following -- to the end of the line is considered a comment.

        -- This is a single-line comment
        SELECT employee_name, salary
        FROM employees
        WHERE department = 'HR';
        

Result:

Multi-line Comments

Multi-line comments start with /* and end with */. Any text between /* and */ is considered a comment.

        /* This is a multi-line comment
           It can span multiple lines */
        SELECT employee_name, salary
        FROM employees
        WHERE department = 'Finance';
        

Result:

Using Comments to Prevent Execution

Comments can also be used to prevent execution of SQL statements. This is useful for debugging purposes.

        -- SELECT * FROM employees;  -- This statement will not be executed
        

Result: