MySql DB

MySQL CROSS JOIN

Last modified on: December 11, 2024

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<h3>MySQL CROSS JOIN: Cartesian Product of Tables</h3>
<p>The <strong><code>CROSS JOIN</code></strong> in MySQL produces a Cartesian product of two tables. This means it combines each row from the first table with every row from the second table, resulting in a potentially large result set.</p>
<hr />
<h3>Syntax</h3>
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<pre class="language-php"><code>SELECT columns
FROM table1
CROSS JOIN table2;
</code></pre>
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<p>Alternatively, you can omit the explicit <code>CROSS JOIN</code> keyword:</p>
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<pre class="language-php"><code>SELECT columns
FROM table1, table2;
</code></pre>
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<hr />
<h3>Key Features of CROSS JOIN</h3>
<ol>
<li>
<p><strong>No Join Condition</strong>:</p>
<ul>
<li>The <code>CROSS JOIN</code> does not use a condition to match rows.</li>
<li>It returns every possible combination of rows from both tables.</li>
</ul>
</li>
<li>
<p><strong>Result Size</strong>:</p>
<ul>
<li>If <code>table1</code> has <em>n</em> rows and <code>table2</code> has <em>m</em> rows, the result will contain <em>n &times; m</em> rows.</li>
</ul>
</li>
<li>
<p><strong>Use Case</strong>:</p>
<ul>
<li>Useful for generating combinations, such as pairing all products with all customers.</li>
</ul>
</li>
</ol>
<hr />
<h3>Example Tables</h3>
<h4><code>employees</code> Table:</h4>
<table>
<thead>
<tr>
<th>employee_id</th>
<th>name</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>John</td>
</tr>
<tr>
<td>2</td>
<td>Alice</td>
</tr>
</tbody>
</table>
<h4><code>departments</code> Table:</h4>
<table>
<thead>
<tr>
<th>department_id</th>
<th>department_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>10</td>
<td>HR</td>
</tr>
<tr>
<td>20</td>
<td>Sales</td>
</tr>
</tbody>
</table>
<hr />
<h3>Example 1: Basic CROSS JOIN</h3>
<p>Retrieve all combinations of employees and departments:</p>
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<pre class="language-php"><code>SELECT employees.name, departments.department_name
FROM employees
CROSS JOIN departments;
</code></pre>
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<h4>Result:</h4>
<table>
<thead>
<tr>
<th>name</th>
<th>department_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>John</td>
<td>HR</td>
</tr>
<tr>
<td>John</td>
<td>Sales</td>
</tr>
<tr>
<td>Alice</td>
<td>HR</td>
</tr>
<tr>
<td>Alice</td>
<td>Sales</td>
</tr>
</tbody>
</table>
<hr />
<h3>Example 2: Using CROSS JOIN with Explicit Columns</h3>
<p>Retrieve combinations and additional calculated fields:</p>
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<pre class="language-php"><code>SELECT e.name AS employee_name, d.department_name
FROM employees AS e
CROSS JOIN departments AS d;
</code></pre>
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<hr />
<h3>Example 3: CROSS JOIN for Generating Test Data</h3>
<h4><code>products</code> Table:</h4>
<table>
<thead>
<tr>
<th>product_id</th>
<th>product_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>Laptop</td>
</tr>
<tr>
<td>2</td>
<td>Phone</td>
</tr>
</tbody>
</table>
<h4><code>customers</code> Table:</h4>
<table>
<thead>
<tr>
<th>customer_id</th>
<th>customer_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>101</td>
<td>Alice</td>
</tr>
<tr>
<td>102</td>
<td>Bob</td>
</tr>
</tbody>
</table>
<p>Generate all product and customer combinations:</p>
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<pre class="language-php"><code>SELECT products.product_name, customers.customer_name
FROM products
CROSS JOIN customers;
</code></pre>
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<h4>Result:</h4>
<table>
<thead>
<tr>
<th>product_name</th>
<th>customer_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>Laptop</td>
<td>Alice</td>
</tr>
<tr>
<td>Laptop</td>
<td>Bob</td>
</tr>
<tr>
<td>Phone</td>
<td>Alice</td>
</tr>
<tr>
<td>Phone</td>
<td>Bob</td>
</tr>
</tbody>
</table>
<hr />
<h3>Example 4: CROSS JOIN with Filters</h3>
<p>Combine <code>CROSS JOIN</code> with a <code>WHERE</code> clause to filter results:</p>
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<pre class="language-php"><code>SELECT employees.name, departments.department_name
FROM employees
CROSS JOIN departments
WHERE departments.department_name = 'HR';
</code></pre>
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</div>
</div>
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<h4>Result:</h4>
<table>
<thead>
<tr>
<th>name</th>
<th>department_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>John</td>
<td>HR</td>
</tr>
<tr>
<td>Alice</td>
<td>HR</td>
</tr>
</tbody>
</table>
<hr />
<h3>Practical Use Cases</h3>
<ol>
<li><strong>Generate Combinations</strong>:
<ul>
<li>Create all possible pairs between two datasets, such as customers and products.</li>
</ul>
</li>
<li><strong>Testing</strong>:
<ul>
<li>Populate datasets for testing scenarios.</li>
</ul>
</li>
<li><strong>Matrix Multiplications</strong>:
<ul>
<li>Use Cartesian products in analytical calculations.</li>
</ul>
</li>
</ol>
<hr />
<h3>Performance Considerations</h3>
<ol>
<li><strong>Large Datasets</strong>:
<ul>
<li>The result set grows exponentially with the size of the tables, so use carefully.</li>
</ul>
</li>
<li><strong>Avoid Unnecessary Usage</strong>:
<ul>
<li>Only use <code>CROSS JOIN</code> when Cartesian products are required.</li>
</ul>
</li>
<li><strong>Apply Filters</strong>:
<ul>
<li>Use <code>WHERE</code> clauses to limit the size of the result set.</li>
</ul>
</li>
</ol>
<hr />
<h3>CROSS JOIN vs Other Joins</h3>
<table>
<thead>
<tr>
<th>Feature</th>
<th>CROSS JOIN</th>
<th>INNER JOIN</th>
<th>LEFT/RIGHT JOIN</th>
</tr>
</thead>
<tbody>
<tr>
<td>Row Matching</td>
<td>Combines every row in both tables</td>
<td>Matches rows based on condition</td>
<td>Includes unmatched rows with <code>NULL</code></td>
</tr>
<tr>
<td>Result Size</td>
<td>Cartesian product</td>
<td>Subset based on match</td>
<td>Includes all rows from one table</td>
</tr>
<tr>
<td>Use Case</td>
<td>Combinations, test data</td>
<td>Related data retrieval</td>
<td>Inclusive data retrieval</td>
</tr>
</tbody>
</table>
<hr />
<h3>&nbsp;</h3>
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