SQL Comments

SQL Comments

SQL comments are used to explain code and improve readability. Comments are ignored by the database engine during execution.

Single-Line Comments

Use two hyphens (--) to create a single-line comment.

-- This query selects all customers
SELECT * FROM customers;

Multi-Line Comments

Use /* */ to create multi-line comments.

/*
  This query retrieves
  active customers only
*/
SELECT *
FROM customers
WHERE status = 'active';

MySQL Special Comment Syntax

MySQL supports version-specific comments using /*! ... */.

SELECT /*!40101 SQL_NO_CACHE */ *
FROM customers;

This syntax executes only on MySQL versions that support the feature.

Comments in Stored Procedures

DELIMITER //

CREATE PROCEDURE ExampleProcedure()
BEGIN
  -- Select all customers
  SELECT * FROM customers;
END //

DELIMITER ;

Best Practices

• Use comments to explain complex queries
• Avoid obvious or redundant comments
• Keep comments updated with code changes

Common Mistakes

• Using incorrect comment syntax
• Forgetting to close multi-line comments
• Leaving outdated comments in production code

Next Step

Continue with SQL Operators to review comparison, logical, and arithmetic operators used in SQL.