In SQL, comments are used to include explanatory notes or disable parts of the code without executing them. Comments can improve code readability and help others understand the logic behind queries. There are two types of comments in SQL: single-line and multi-line.

Single-line Comments
Single-line comments start with two hyphens (–). Anything after these hyphens on the same line will be ignored by the SQL engine.

Example:

— This is a single-line comment
SELECT ProductName, Price
FROM Products; — This comment is ignored by SQL

Multi-line Comments
Multi-line comments start with /* and end with */. They can span multiple lines, making them useful for longer explanations or temporarily disabling large blocks of code.

Example:

/*
This is a multi-line comment.
It spans multiple lines.
*/
SELECT ProductName, Price
FROM Products;

Another Example:
SELECT ProductName, Price /* This comment is ignored by SQL */
FROM Products;

 

Using Comments for Documentation
Comments can be very useful for documenting the purpose and functionality of your SQL queries. This practice is especially important in complex queries or when working in a team.

Example:

— Select product name and price from the Products table
— This query fetches all products with their prices
SELECT ProductName, Price
FROM Products;

Using Comments to Disable Code
You can use comments to temporarily disable parts of your SQL code for testing or debugging purposes.

Example:

SELECT ProductName, Price
FROM Products
— WHERE Price > 1.00; — This condition is disabled

Best Practices for Using Comments
Keep Comments Relevant and Up-to-date: Ensure that comments accurately describe the code. Outdated comments can be misleading.
Be Clear and Concise: Write comments that are easy to understand. Avoid unnecessary verbosity.
Use Comments to Explain Why, Not What: Focus on explaining the reasoning behind the code rather than describing what the code does. The code itself should be self-explanatory when possible.
Example:

— Fetch all products with a price greater than 1.00
— This helps in identifying premium products
SELECT ProductName, Price
FROM Products
WHERE Price > 1.00;