SQL Comments Tutorial

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);
        

employee_id	employee_name	department	salary
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:

employee_name	John Doe
salary	50000.00
employee_name	Nancy White
salary	55000.00

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:

employee_name	Jane Smith
salary	60000.00
employee_name	Mike Green
salary	65000.00

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:

employee_id
employee_name
department
salary