SQL Comments

A comment is text inside a SQL statement that the database ignores entirely, meant purely for whoever reads the query later. Every lesson's code examples in this course have used them already, this lesson covers the syntax directly.

Learning Objectives

• Write a single-line comment with --.
• Write a multi-line block comment with /* */.
• Know the one extra comment style specific to MySQL and MariaDB.

Single-Line Comments

-- starts a comment that runs to the end of the line, works identically across every database in this course, and can appear on its own line or trailing after real SQL on the same line.

-- Only return employees earning over 90,000
SELECT first_name, salary
FROM employees
WHERE salary > 90000; -- filters after the join, not before

Block Comments

/* starts a block comment, * / ends it, spanning any number of lines in between, also identical across every database here. Block comments can also appear mid-statement, around a single clause, not just surrounding the whole query.

/*
  This query powers the quarterly budget report.
  Last reviewed: see Module 3, AVG() lesson.
*/
SELECT department_id, AVG(salary) AS average_salary
FROM employees
GROUP BY department_id;

MySQL and MariaDB's Extra Style

Try It Yourself

Add a single-line comment explaining why this query filters on department_id, and a block comment above it noting who it's for.

/*
  For the HR team's Engineering headcount review.
*/
SELECT first_name
FROM employees
WHERE department_id = 1; -- Engineering only, see joins-overview lesson for the schema

Frequently Asked Questions

Common Beginner Mistakes

Best Practices

• Use -- for short, single-line notes, and /* */ for longer explanations spanning multiple lines.
• Avoid # for anything that might ever run on a database other than MySQL/MariaDB.
• Comment the why, not the what, a query's syntax already shows what it does, a comment is most useful explaining a non-obvious reason.

Interview Questions

Summary

-- (single-line) and /* */ (block) are portable comment syntax across every database in this course. # is an extra single-line style specific to MySQL and MariaDB. Comments are stripped before execution and have no effect on performance.

What's Next?

This module closes with a full SQL Operators Reference, pulling comparison, logical, and arithmetic operators together in one place, including a few, like modulo and string concatenation, not covered yet.