SQL comments let you document a query or temporarily disable part of it without deleting code. They're ignored by the database engine.
Single-Line Comments
-- This gets every employee earning above 50,000
SELECT * FROM employees WHERE salary > 50000;Everything from -- to the end of the line is ignored. Note: standard SQL requires a space after --.
Multi-Line Comments
/* This query powers the monthly payroll report.
Do not remove the status filter — finance relies on it. */
SELECT name, salary
FROM employees
WHERE status = 'active';Dialect Note: The # Symbol
MySQL only also supports # for a single-line comment:
# MySQL-specific single-line comment
SELECT * FROM employees;PostgreSQL and SQL Server do not support # as a comment marker — stick to -- for anything you want to be portable across databases.
Practical Use Cases
- Explaining why a query filters a certain way (business context a future reader won't guess)
- Temporarily commenting out a clause while debugging a large query
- Leaving a TODO for a report that needs revisiting
Common Mistake
Over-commenting obvious code (e.g. -- select the name column above SELECT name). Comment the business reason, not the syntax — the syntax is already readable.
Interview Relevance
Rarely asked directly, but expect to be judged on whether your interview SQL is readable — a well-placed comment on a non-obvious filter can make a good impression during a live SQL round.
Practice Question
Add a one-line comment explaining why this filter exists: WHERE order_date >= '2024-01-01' AND status != 'cancelled'
