SQL Comments

🔹 What Are SQL Comments?

A comment in SQL is text written inside your SQL code that the database ignores when executing the query.

Think of comments as:

📝 Notes for humans (developers)

❌ Ignored by the database

They help explain:

• What the query does
• Why something was written a certain way
• Important reminders
• Temporary disabling of code

🔹 Why Are SQL Comments Important?

Comments help:

• Make code easier to read
• Help other developers understand your work
• Debug queries
• Temporarily disable parts of code

🔹 Types of SQL Comments

There are two main types of SQL comments:

| Comment Type        | Symbol Used | Purpose |
|---------------------|-------------|----------|
| Single-line comment | `--`        | Comment one line |
| Multi-line comment  | `/* */`     | Comment multiple lines |

🔹 1️⃣ Single-Line Comments (--)

Single-line comments start with:

--

Everything after -- on that line is ignored.

Example:

-- This query selects all employees
SELECT * FROM Employees;

You can also write it at the end of a line:

SELECT * FROM Employees; -- Get all employee records

🔹 2️⃣ Multi-Line Comments (/* */)

Multi-line comments start with:

/*

and end with:

*/

Everything inside is ignored.

Example:

/*
This query retrieves employees
from the HR department
*/
SELECT * 
FROM Employees
WHERE department = 'HR';

🔹 Example Table (Markdown Format)

Here’s a sample Employees table:

| id | name    | department | salary |
|----|---------|------------|--------|
| 1  | Alice   | HR         | 50000  |
| 2  | Bob     | Sales      | 60000  |
| 3  | Charlie | IT         | 55000  |
| 4  | David   | HR         | 45000  |

🔹 Commenting Complex Queries

Comments are especially helpful in long queries.

Example:

-- Get high-paying HR employees
SELECT id, name, salary
FROM Employees
WHERE department = 'HR'
AND salary > 48000;

🔹 Using Comments to Disable Code (Very Useful!)

You can temporarily disable part of a query.

Example:

SELECT id, name, salary
FROM Employees
WHERE department = 'HR'
-- AND salary > 48000;

The salary condition is now ignored.

Or disable multiple lines:

SELECT id, name, salary
FROM Employees
/*
WHERE department = 'HR'
AND salary > 48000
*/
;

🔹 Best Practices for SQL Comments

| Best Practice | Explanation |
|---------------|-------------|
| Explain WHY, not WHAT | Avoid obvious comments like "Select all columns" |
| Keep comments clear | Short and meaningful |
| Update comments | Make sure comments match your code |
| Use for complex logic | Especially in joins and conditions |

🔹 Real-World Example

/*
Monthly HR Salary Report
Author: Admin
Purpose: Retrieve HR employees earning above 45,000
*/
SELECT id, name, salary
FROM Employees
WHERE department = 'HR'
AND salary > 45000;