SQL Select Distinct

Last modified on: December 2, 2024

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<h3><strong>SQL SELECT DISTINCT: A Complete Guide</strong></h3>
<h4><strong>What is SELECT DISTINCT?</strong></h4>
<p>The <strong><code>SELECT DISTINCT</code></strong> statement is used to retrieve unique values from a specified column in a database table. It eliminates duplicate rows in the result set, ensuring that each value is displayed only once.</p>
<p>This is especially useful when you want to analyze or display only unique data, such as unique categories, cities, or product names.</p>
<hr />
<h4><strong>Syntax</strong></h4>
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<pre class="language-php"><code>SELECT DISTINCT column1, column2, ...
FROM table_name
WHERE condition;
</code></pre>
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<h3><strong>Examples of SELECT DISTINCT Queries</strong></h3>
<h4><strong>1. Retrieve Unique Values from a Single Column</strong></h4>
<p>Suppose we have a <code>products</code> table:</p>
<p><strong>Table: <code>products</code></strong></p>
<table>
<thead>
<tr>
<th>id</th>
<th>name</th>
<th>category</th>
<th>price</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>Laptop</td>
<td>Electronics</td>
<td>1200</td>
</tr>
<tr>
<td>2</td>
<td>Smartphone</td>
<td>Electronics</td>
<td>800</td>
</tr>
<tr>
<td>3</td>
<td>Refrigerator</td>
<td>Appliances</td>
<td>1500</td>
</tr>
<tr>
<td>4</td>
<td>Smartphone</td>
<td>Electronics</td>
<td>800</td>
</tr>
</tbody>
</table>
<p><strong>Query: Retrieve unique categories</strong></p>
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<pre class="language-php"><code>SELECT DISTINCT category 
FROM products;
</code></pre>
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<p><strong>Output:</strong></p>
<table>
<thead>
<tr>
<th>category</th>
</tr>
</thead>
<tbody>
<tr>
<td>Electronics</td>
</tr>
<tr>
<td>Appliances</td>
</tr>
</tbody>
</table>
<hr />
<h4><strong>2. Retrieve Unique Combinations of Multiple Columns</strong></h4>
<p>Fetch unique combinations of category and price:</p>
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<pre class="language-php"><code>SELECT DISTINCT category, price 
FROM products;
</code></pre>
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<p><strong>Output:</strong></p>
<table>
<thead>
<tr>
<th>category</th>
<th>price</th>
</tr>
</thead>
<tbody>
<tr>
<td>Electronics</td>
<td>1200</td>
</tr>
<tr>
<td>Electronics</td>
<td>800</td>
</tr>
<tr>
<td>Appliances</td>
<td>1500</td>
</tr>
</tbody>
</table>
<hr />
<h4><strong>3. Use DISTINCT with WHERE Clause</strong></h4>
<p>Retrieve unique product categories where price is greater than $1000:</p>
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<pre class="language-php"><code>SELECT DISTINCT category 
FROM products 
WHERE price &gt; 1000;
</code></pre>
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<p><strong>Output:</strong></p>
<table>
<thead>
<tr>
<th>category</th>
</tr>
</thead>
<tbody>
<tr>
<td>Electronics</td>
</tr>
<tr>
<td>Appliances</td>
</tr>
</tbody>
</table>
<hr />
<h4><strong>4. Combine DISTINCT with ORDER BY</strong></h4>
<p>Retrieve unique product names and sort them alphabetically:</p>
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<pre class="language-php"><code>SELECT DISTINCT name 
FROM products 
ORDER BY name ASC;
</code></pre>
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<p><strong>Output:</strong></p>
<table>
<thead>
<tr>
<th>name</th>
</tr>
</thead>
<tbody>
<tr>
<td>Laptop</td>
</tr>
<tr>
<td>Refrigerator</td>
</tr>
<tr>
<td>Smartphone</td>
</tr>
</tbody>
</table>
<hr />
<h4><strong>5. Using DISTINCT in Aggregate Functions</strong></h4>
<p>You can&rsquo;t directly use DISTINCT with an aggregate function, but it&rsquo;s often part of the logic. For example, finding the total distinct categories:</p>
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<pre class="language-php"><code>SELECT COUNT(DISTINCT category) AS distinct_categories 
FROM products;
</code></pre>
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<p><strong>Output:</strong></p>
<table>
<thead>
<tr>
<th>distinct_categories</th>
</tr>
</thead>
<tbody>
<tr>
<td>2</td>
</tr>
</tbody>
</table>
<hr />
<h3><strong>Tips for Using SELECT DISTINCT</strong></h3>
<ol>
<li><strong>Use Judiciously</strong>: Applying <code>DISTINCT</code> can be computationally expensive on large datasets; ensure it&rsquo;s necessary for your query.</li>
<li><strong>Sort Results</strong>: Combine <code>DISTINCT</code> with <code>ORDER BY</code> for better data presentation.</li>
<li><strong>Avoid Redundancy</strong>: If your table design already ensures uniqueness (e.g., using primary keys), <code>DISTINCT</code> might be unnecessary.</li>
</ol>
<hr />
<h3>&nbsp;</h3>
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