Which are valid SQL comments?
Assesses fundamental understanding of SQL & Databases conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
SQL supports both single-line and multi-line commenting syntax across relational database engines:
### 1. Single-Line Standard Comment (--):
The official ANSI SQL standard single-line comment begins with two hyphens followed by a space:
-- Query active users who signed up this year
SELECT * FROM users WHERE signup_year = 2026;
### 2. Multi-Line Comment (/* ... */):
Used for extended explanations or commenting out blocks of SQL logic:
/*
Author: Engineering Team
Purpose: Calculate month-end churn metrics
*/
SELECT department, COUNT(*) FROM employees GROUP BY department;
### 3. MySQL-Specific Hash Comment (#):
MySQL also supports the hash symbol (#) for single-line comments, though -- is strongly preferred for ANSI portability.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.