SQL Comments

Last modified on: December 3, 2024

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<h3><strong>SQL Comments: A Complete Guide</strong></h3>
<h4><strong>What are SQL Comments?</strong></h4>
<p>SQL comments are used to add explanations, notes, or annotations to SQL code. They are ignored by the database engine during execution. Comments are helpful for improving code readability, documenting logic, and debugging.</p>
<hr />
<h3><strong>Types of SQL Comments</strong></h3>
<p>SQL supports two types of comments:</p>
<ol>
<li><strong>Single-line Comments</strong></li>
<li><strong>Multi-line Comments</strong></li>
</ol>
<hr />
<h3><strong>1. Single-Line Comments</strong></h3>
<p>Single-line comments start with <code>--</code> and continue until the end of the line. These are useful for short notes or explanations.</p>
<h4><strong>Syntax:</strong></h4>
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<pre class="language-php"><code>-- This is a single-line comment
SELECT * FROM employees; -- This query retrieves all employees
</code></pre>
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<h4><strong>Example:</strong></h4>
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<pre class="language-php"><code>-- Fetch all records from the employees table
SELECT * FROM employees;
</code></pre>
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<h3><strong>2. Multi-Line Comments</strong></h3>
<p>Multi-line comments start with <code>/*</code> and end with <code>*/</code>. They can span multiple lines, making them ideal for longer descriptions or temporary code blocks.</p>
<h4><strong>Syntax:</strong></h4>
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<pre class="language-php"><code>/* 
   This is a multi-line comment.
   Use it to explain complex logic or temporarily disable code.
*/
SELECT * FROM employees;
</code></pre>
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<h4><strong>Example:</strong></h4>
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<pre class="language-php"><code>/*
   Retrieve the total salary for each department.
   This query uses GROUP BY to aggregate data by department.
*/
SELECT department, SUM(salary) AS total_salary
FROM employees
GROUP BY department;
</code></pre>
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<h3><strong>Using Comments for Debugging</strong></h3>
<p>You can temporarily disable parts of your SQL code using comments.</p>
<h4><strong>Example:</strong></h4>
<p>Before debugging:</p>
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<pre class="language-php"><code>SELECT employee_id, employee_name, salary
FROM employees
WHERE salary &gt; 50000;
</code></pre>
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<p>Disable the condition for debugging:</p>
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<pre class="language-php"><code>SELECT employee_id, employee_name, salary
FROM employees;
/* WHERE salary &gt; 50000; */
</code></pre>
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<h3><strong>Best Practices for SQL Comments</strong></h3>
<ol>
<li><strong>Explain the Purpose of Queries:</strong> Add comments to describe what the query is doing.</li>
<li><strong>Keep It Concise:</strong> Avoid unnecessary or overly verbose comments.</li>
<li><strong>Use Multi-Line Comments for Detailed Explanations:</strong> For complex logic, use multi-line comments to provide clarity.</li>
<li><strong>Document Assumptions and Parameters:</strong> If a query relies on specific conditions or assumptions, document them.</li>
<li><strong>Regularly Review Comments:</strong> Ensure comments are updated when code changes.</li>
</ol>
<hr />
<h3><strong>Examples of Comment Usage</strong></h3>
<h4><strong>1. Documenting Query Steps</strong></h4>
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<pre class="language-php"><code>-- Step 1: Fetch all employees in the IT department
SELECT employee_id, employee_name
FROM employees
WHERE department = 'IT';
</code></pre>
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<h4><strong>2. Explaining Conditional Logic</strong></h4>
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<pre class="language-php"><code>/* Retrieve employees with a salary greater than 50,000.
   Exclude those in the 'Intern' job title. */
SELECT employee_name, salary
FROM employees
WHERE salary &gt; 50000 AND job_title != 'Intern';
</code></pre>
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<h4><strong>3. Temporarily Disabling Code</strong></h4>
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<pre class="language-php"><code>-- Disable this query while testing
-- SELECT * FROM employees;
</code></pre>
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<h3>&nbsp;</h3>
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