MySQL Comments

Why Comment Your SQL

Queries that look obvious the day you write them can be baffling months later, especially once WHERE clauses, subqueries, and joins pile up. Comments let you record why a query is shaped the way it is — business rules, edge cases, or reminders for whoever maintains it next, often yourself.

Single-Line Comments with --

A double dash starts a comment that runs to the end of the line. MySQL requires at least one whitespace character, a space or tab, after the two dashes for it to be recognized as a comment — writing two dashes directly followed by text with no space can be treated differently and cause a syntax error, so always include the space.

• -- single line comment (MySQL requires a space after the dashes)
• # single line comment, a MySQL-specific shorthand that needs no trailing space
• /* ... */ block comment that can span multiple lines or sit mid-statement

-- Get all active customers who joined this year
SELECT customer_id, name, join_date
FROM customers
WHERE status = 'active'          -- only active accounts
  AND join_date >= '2024-01-01';  -- new customers only

Block comments open with /* and close with */, and unlike -- or #, they can stretch across several lines or even sit in the middle of a line of code, which makes them useful for longer explanations or for temporarily disabling a chunk of SQL.

SELECT product_name, price
FROM products
/*
WHERE category = 'Electronics'
  AND price > 100
*/
ORDER BY price DESC;

SELECT name, salary FROM employees
# TODO: revisit this threshold after the Q3 salary review
WHERE salary > 50000;