Comments in SQL

Mastering Comments in SQL: Enhancing Code Readability and Maintainability

Comments are used to annotate the code, providing explanations or reminders within the script itself, which do not affect the execution of the code. Understanding how to effectively use comments in SQL is crucial for both newcomers and experienced professionals. Comments make SQL scripts readable and maintainable, whether you are working solo or as part of a team.

Types of Comments in SQL

SQL supports two primary types of comments:

  • Single-line Comments
  • Multi-line Comments

1. Single-line Comments

Single-line comments start with --. Everything following the -- on the same line is treated as a comment and is not executed by SQL.

Example of Single-line Comments:

-- This is a single-line comment
SELECT * FROM Employees; -- This query retrieves all records from the Employees table

In the example above, the first line is a standalone comment. It explains the code’s intention without affecting the function. The second comment is inline with the SQL command, useful for brief explanations or notes directly adjacent to the code.

Practical Usage:

Single-line comments are perfect for short annotations or explanations. They are also handy for temporarily disabling certain parts of SQL code, such as during debugging or when running specific parts of a script:

-- SELECT Name FROM Employees; -- Temporarily disabled to test performance without this line
SELECT ID, Salary FROM Employees; -- This line is still active

2. Multi-line Comments

Multi-line comments start with /* and end with */. Everything between these markers is commented out and ignored by the SQL execution engine.

Example of Multi-line Comments:

    This is a multi-line comment.
    It can span multiple lines.
    Below is a query that retrieves all records from the Employees table.
SELECT * FROM Employees;

Multi-line comments are ideal for longer descriptions or complex explanations that span multiple lines. They are also useful for commenting out large sections of code during testing or debugging.

Practical Usage:

In development environments, multi-line comments can explain complex queries or logic, detail modifications made by different team members, or provide important warnings or instructions to anyone reading the code:

    Changes made on 2024-04-24 by John Doe:
    Removed unnecessary JOIN to improve query performance.
    Note: Ensure compatibility with the Reporting module.
SELECT * FROM Employees WHERE DepartmentID = 3;

Best Practices for Commenting SQL Code

Clarity and Relevance:

Ensure that comments are clear and relevant. They should help anyone reading the code to understand the purpose and logic of the SQL statements without needing to guess.


Keep comments up-to-date as the code evolves. Outdated comments can mislead and detract from the code’s utility.

Use Comments to Explain ‘Why’, Not ‘What’:

Focus on explaining why certain decisions were made in the code, especially if the SQL involves complex logic or unique solutions to problems, rather than simply reiterating what the code does.

Avoid Over-Commenting:

Too many comments can clutter the code and make it difficult to read. Comment only when necessary to add context or explain complex parts of the code.

Use Inline Comments Sparingly:

While inline comments are useful, excessive use can make lines too long and hard to read. Use them judiciously for brief, pertinent annotations.


Comments are an invaluable tool for improving the readability and maintainability of SQL code. They not only help document the purpose and functionality of your queries but also assist other developers who might work on the same codebase in the future. Effective use of comments can significantly enhance the clarity of the development process, ensuring that all participants understand the code, leading to more efficient and error-free programming.