SQL Practice Online

How to Comment in SQL: A Beginner’s Guide

By Cristian G. Guasch • Updated: 05/18/23 • 7 min read

When it comes to working with SQL databases, commenting can be an essential tool for developers and database administrators alike. Comments in SQL allow you to add notes to your code, explain sections of your queries, and prevent certain lines of code from executing. By adding comments to your SQL code, you can make it more readable, maintainable, and easier to understand for yourself and others.

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SQL comments come in two forms: single-line comments and multi-line comments (also known as block comments). Single-line comments begin with two dashes (–), while multi-line comments are enclosed in forward slash and asterisk (/* and */). Both types of comments can be used to document your SQL code, but multi-line comments are generally better suited for longer explanations or comments that span multiple lines. Additionally, some SQL dialects, such as MySQL, also allow the use of the hash symbol (#) for single-line comments.

What is SQL Comment?

SQL Comment is a feature that allows developers to add notes and explanations to their SQL code without affecting the execution of the code. Comments can be used to describe the purpose of the code, explain complex queries, or simply add reminders to the developer.

SQL Comments Syntax

In SQL, comments are created by starting the comment with two dashes (–) for single-line comments, or by enclosing the comment between /* and */ for multi-line comments. Here’s an example of a single-line comment:

SELECT * FROM customers -- This is a comment

And here’s an example of a multi-line comment:

/*
This is a multi-line comment
It can span multiple lines
*/

Types of SQL Comments

There are two types of SQL comments: single-line comments and multi-line comments.

Single-line Comments

Single-line comments are used to add comments to a single line of code. They start with two dashes (–) and continue until the end of the line. Here’s an example:

SELECT * FROM customers -- This is a comment

Single-line comments are useful for adding short comments to a line of code.

Multi-line Comments

Multi-line comments are used to add comments that span multiple lines of code. They start with /* and end with */. Here’s an example:

/*
This is a multi-line comment
It can span multiple lines
*/

Multi-line comments are useful for adding longer comments that explain complex code.

In conclusion, SQL comments are a valuable feature of SQL that allows developers to add notes and explanations to their code without affecting the execution of the code. Single-line and multi-line comments are the two types of comments that can be used in SQL, and they can be used to add short or long comments to the code.

Why Use SQL Comments?

SQL comments are essential for any developer who wants to write clean, readable, and maintainable code. Here are some reasons why developers should use SQL comments:

1. Explain Code Sections

Comments are used to explain sections of SQL statements. They help developers understand what a particular section of code does. For example, a comment can be added to explain a complex SELECT statement. This makes it easier for other developers to read and understand the code.

2. Prevent Execution of Code

Comments can also be used to prevent the execution of SQL statements. For example, if a developer wants to test a new SQL statement without deleting the existing code, they can comment out the old code. This ensures that the old code is not executed when the SQL statement is run.

3. Improve Code Readability

SQL comments improve code readability. A well-commented code is easier to read and understand. This is especially useful when working on large projects with multiple developers. Comments also help developers remember why they wrote a particular piece of code.

4. Document Code Changes

Comments can be used to document code changes. When a developer makes changes to an existing SQL statement, they can add a comment to explain what they changed and why. This helps other developers understand the changes and why they were made.

5. Best Practice

Using comments in SQL is considered a best practice. It is a way to ensure that the code is readable, maintainable, and understandable. SQL comments are also useful when debugging code. Developers can use comments to isolate a particular section of code and test it separately.

In conclusion, SQL comments are an essential tool for any developer who wants to write clean, readable, and maintainable code. They help explain code sections, prevent execution of code, improve code readability, document code changes, and are considered a best practice in SQL development.

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How to Comment in SQL

When working with SQL, it’s important to add comments to your code to make it more understandable and maintainable. Comments in SQL are used to explain the purpose of the code, add notes, and provide context to other developers who may read the code. In this section, we’ll cover the different ways to add comments in SQL.

Single-Line Comments

Single-line comments in SQL are used to comment out a single line of code. To add a single-line comment, use two dashes “–” followed by the comment text. Anything after the two dashes will be ignored by the SQL engine. Here’s an example:

SELECT * FROM Customers -- This is a comment

In the above example, the SQL engine will only execute the SELECT statement and ignore the comment text after the two dashes.

Multi-Line Comments

Multi-line comments in SQL are used to comment out multiple lines of code. To add a multi-line comment, use “/” to start the comment and “/” to end the comment. Anything between the “/” and “/” will be ignored by the SQL engine. Here’s an example:

/*
This is a multi-line comment
that spans across multiple lines
*/
SELECT * FROM Customers

In the above example, the SQL engine will ignore the entire multi-line comment and only execute the SELECT statement.

Uncommenting SQL Code

To uncomment SQL code, simply remove the comment syntax from the code. For single-line comments, delete the two dashes “–” and the comment text. For multi-line comments, delete the “/” and “/” and the comment text in between.

It’s important to note that nested comments are not supported in SQL. If you try to nest comments, you’ll get a syntax error.

In summary, comments in SQL are a powerful tool for making your code more understandable and maintainable. By using single-line and multi-line comments, you can add context and notes to your code. When commenting out code, use the appropriate syntax for single-line and multi-line comments. And remember, nested comments are not supported in SQL.

Best Practices for SQL Comments

When writing SQL code, it is important to include comments to improve readability and maintainability. Here are some best practices to follow when adding comments to your SQL code:

Use descriptive comments

Comments should provide context and explain the purpose of the code. Use descriptive comments that explain the logic behind the code. This will help other developers understand the code and make it easier to maintain in the future. For example, instead of using a comment like -- get data, use -- retrieve all student email addresses from the database.

Use consistent formatting

Consistent formatting makes code easier to read and understand. Use the same formatting for all comments in your code. For example, if you use block comments (/* */) for one comment, use them for all comments in your code.

Avoid unnecessary comments

While comments are helpful, too many comments can make code difficult to read. Avoid commenting on obvious or self-explanatory code. Only add comments when they add value to the code.

Use single-line comments for short comments

Single-line comments (--) are useful for short comments that fit on one line. Use single-line comments to explain specific lines of code or to add context to a query. For example, -- join student and email tables on student ID or -- filter results to only include students who were last updated within the last month.

Use block comments for longer comments

Block comments (/* */) are useful for longer comments that span multiple lines. Use block comments to explain the purpose of a stored procedure or function, or to provide an overview of a complex query. For example:

/*
This stored procedure retrieves all student email addresses from the database
and returns them as a comma-separated list.
*/

By following these best practices, you can make your SQL code easier to read and understand, which will save time and effort in the long run.

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