Comments in MySQL

Comments in MySQL are used to add explanations or disable parts of queries without executing them. MySQL supports three types of comments:

1. Single-Line Comments (-- or #)

• Used for short explanations or debugging.
• Everything after -- (double hyphen with a space) or # is ignored by MySQL.

Example: Using --

SELECT * FROM employees;  -- This fetches all employees

✅ Note: There must be a space after -- (--⎵).
❌ --This is wrong (No space after --)

Example: Using #

SELECT * FROM employees;  # This is also a valid comment

2. Multi-Line Comments (/* ... */)

• Used for long comments or disabling multiple lines of code.

Example: Commenting out a block

/*  This query retrieves employees  from the IT department only.*/SELECT * FROM employees WHERE department = 'IT';

Example: Disabling a part of a query

SELECT * FROM employees WHERE department = 'IT' /* AND salary > 50000 */ORDER BY employee_name;

✅ The part inside /* ... */ is ignored.

3. Nested Comments (MySQL-Specific)

• MySQL supports nested comments using /*! ... */.
• Used for hints or version-specific code.

Example: Executed only in MySQL

SELECT * FROM employees /*! WHERE salary > 50000 */;

✅ Runs as normal SQL in MySQL.
❌ Ignored in other SQL databases.

Best Practices

✅ Use -- for quick inline notes.
✅ Use /* ... */ for long explanations or disabling code.
✅ Avoid excessive commenting in production queries.