When working with SQL, clarity and maintainability are just as important as writing efficient and functional queries. Whether you’re crafting data-intensive stored procedures, querying across several joins, or just pulling reports, how you document your code can either save time or waste hours for your future self and your teammates. That’s where SQL comments come into play.
TL;DR
Good SQL comments help explain the purpose, logic, and structure of your queries, making collaboration smoother and reducing future debugging time. Avoid over-commenting by focusing on non-obvious logic or business rules. Use consistent formatting for better readability, and keep comments up to date as your SQL logic evolves. Comments should clarify code, not replace clean design—use them wisely.
Why Comments in SQL Matter
Anyone who’s ever had to debug a tricky JOIN or a nested subquery knows how painful legacy queries can be—especially un-commented ones. With SQL, we operate closer to business data, meaning the logic often hides assumptions and rules that aren’t visible in syntax alone. This is where strategic and readable commenting becomes a vital tool.
Think of commenting like leaving signposts for your future self—or for a new team member joining the project.
Types of SQL Comments
SQL supports two main types of comments:
- Single-line comments: These begin with
--and extend to the end of the line. - Multi-line comments: Enclosed between
/*and*/, these allow for block-level documentation.
Example:
-- This is a single-line comment
/*
This is a multi-line comment
explaining the following JOIN logic.
*/Best Practices for Commenting in SQL
Now that we know how to comment, let’s look at how to do it effectively. Commenting your SQL should add value and clarity instead of clutter and confusion. The following best practices are designed to help you and others understand and maintain SQL code over time.
1. Comment the “Why”, Not the “What”
This is the cardinal rule of code commenting in any language. Don’t waste time stating the obvious. Instead, explain business rules, performance hacks, or rationale behind certain design decisions.
Example:
-- We only include active customers to avoid skewing churn rates
SELECT * FROM Customers WHERE IsActive = 1;2. Keep Comments Up to Date
Outdated comments are worse than no comments. They mislead anyone trying to understand your logic. If you change the logic, update the comment—or it’ll do more harm than good.
3. Use Consistent Formatting
Maintain a uniform style that separates blocks of logic. For example, use lines of hyphens or stars for section headers in long queries or stored procedures.
-- ---------------------------------------
-- Step 2: Aggregate sales by region
-- ---------------------------------------4. Avoid Over-Commenting
While comments are essential, excessive explanations of obvious SQL syntax make your script harder to read. Trust that most SQL developers know what a SELECT or JOIN does.
Bad example:
-- Selecting all columns from the Orders table
SELECT * FROM Orders;5. Comment Complex Joins and Subqueries
Nested subqueries and joins can get mind-numbingly complex. Always add a quick note about what each part is doing if the logic isn’t self-evident.
-- Matching only orders where total exceeds average in customer segment
SELECT o.*
FROM Orders o
JOIN (
SELECT CustomerID, AVG(Total) AS AvgOrder
FROM Orders
GROUP BY CustomerID
) a ON a.CustomerID = o.CustomerID AND o.Total > a.AvgOrder;6. Add Section Comments in Long Scripts
For stored procedures or multi-part queries, use comments to break your script into readable sections. This improves navigation and comprehension.
-- ==============================
-- Part 1: Data cleanup
-- ==============================7. Document Temporary Tables and CTEs
Common Table Expressions (CTEs) and temporary tables often wrap key logic. Explain their role, filters, or why they’re needed before defining them.
-- CTE to get latest orders for each customer
WITH LatestOrders AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY CustomerID ORDER BY OrderDate DESC) AS rn
FROM Orders
)
SELECT * FROM LatestOrders WHERE rn = 1;8. Indicate Areas of Potential Risk or Technical Debt
Sometimes you have to take shortcuts due to timeline pressures. Mark these areas clearly so they can be revisited later.
-- TODO: Replace hardcoded dates with dynamic parameters
-- Currently filtering data from Q1 2023 only
WHERE OrderDate BETWEEN '2023-01-01' AND '2023-03-31';Real-World Scenarios Where Good Comments Shine
1. Onboarding New Developers: Well-commented SQL helps shorten the learning curve for new team members, allowing them to quickly understand the data logic and constraints specific to your environment.
2. Debugging in Production: When a report starts showing incorrect numbers, deciphering unfamiliar queries becomes easier if the purpose of each clause has been explained in brief, targeted comments.
3. Legacy System Maintenance: Legacy SQL codebases can contain cryptic queries that have evolved over the years. In such environments, comments serve as a historical record of thought processes and assumptions.
Commenting in SQL Tools and Environments
Different SQL environments interpret or display comments in their own ways. For example:
- SSMS (SQL Server Management Studio): Ignores comments during execution but displays them in the query editor for easy reference.
- MySQL Workbench: Fully supports both single and multi-line comments.
- Oracle SQL Developer: Allows block comments but warns if used improperly inside PL/SQL blocks.
It’s good to test how comments show up—or don’t—especially when writing SQL to be exported, shared, or used in automated scripts.
Tools That Help With Commenting
Several tools and extensions can assist you in keeping SQL well-documented:
- SQL Code Formatters: Many offer options to auto-align comments with code for improved readability.
- Linting Tools: Plugins like SQLFluff can flag missing or misplaced comments and suggest additions.
- Version Control Hooks: Some teams implement checks to ensure business-critical SQL includes explanatory comments before being accepted into main branches.
The Downside of Poor Commenting
Let’s be honest—a lot of SQL in the wild has zero comments and poor naming conventions. This eventually costs teams hours in debugging and on-boarding time. Here’s what can go wrong:
- Confusion over why logic exists
- Misinterpretation of business rules
- Longer testing and review cycles
- Mistakes in modification due to misunderstanding existing logic
When clarity disappears, bugs multiply.
Conclusion: Comments Are Part of Writing “Clean” SQL
Good SQL isn’t just about optimization and security—it also includes legibility. Comments make your intent known, your queries maintainable, and your analytics more trustworthy. Use them with intention and discipline and you’ll help ensure that your SQL doesn’t just run—it communicates.
Remember: well-commented SQL is both a gift and a responsibility. Make it easier for yourself and your team to understand, debug, and enhance queries over time. Let your comments reflect your professionalism and care for clean, transparent code.