SQL Comments

SQL has two comment styles. The parser ignores them, but every reviewer reads them.

Single-line comment

-- This is a single-line comment
SELECT * FROM customers;  -- you can also put one at end of line

Block comment

/*
  Daily revenue report.
  Owner: @ada
  Last touched: 2026-06-06
*/
SELECT DATE(created_at) AS day, SUM(total) AS revenue
FROM orders
GROUP BY DATE(created_at);

MySQL extension — #

MySQL accepts # as a single-line comment, too. Stick with -- for portability.

Comment-out trick

Wrap a clause in a block comment to quickly disable it for testing:

SELECT *
FROM customers
WHERE active = 1
  /* AND country = 'AU' */
ORDER BY name;

What to comment

• Why, not what — the SQL already says what.
• Non-obvious filters: "exclude test accounts created before launch".
• Vendor quirks: "MySQL bug 12345 — must use STRAIGHT_JOIN here".
• Owner and last-touched date for ad-hoc reports.

Example

-- Single-line comment
SELECT * FROM customers; /* inline */
/* Block
   comment */

Exercise

Test yourself