SQL Left Join

Last modified on: December 3, 2024

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<h3><strong>SQL LEFT JOIN (or LEFT OUTER JOIN): A Complete Guide</strong></h3>
<h4><strong>What is a LEFT JOIN?</strong></h4>
<p>A <strong>LEFT JOIN</strong> (also known as a <strong>LEFT OUTER JOIN</strong>) is a type of SQL join that returns all rows from the <strong>left table</strong> and the matched rows from the <strong>right table</strong>. If there is no match, the result will contain <strong>NULL</strong> values for columns from the right table.</p>
<p>In other words, it returns all the rows from the <strong>left table</strong> and only the rows from the <strong>right table</strong> where there is a match based on the join condition. If there&rsquo;s no match, the result will still show the row from the left table with <strong>NULL</strong> for the right table's columns.</p>
<hr />
<h3><strong>Syntax of LEFT JOIN</strong></h3>
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<pre class="language-php"><code>SELECT column_names
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;
</code></pre>
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<ul>
<li><strong>table1</strong>: The left table.</li>
<li><strong>table2</strong>: The right table.</li>
<li><strong>column_name</strong>: The column used to match rows between the two tables.</li>
</ul>
<p>You can also select specific columns from both tables by specifying them in the <code>SELECT</code> clause.</p>
<hr />
<h3><strong>Example of LEFT JOIN</strong></h3>
<h4><strong>Scenario: Joining <code>employees</code> and <code>departments</code> tables</strong></h4>
<p>Let&rsquo;s say you have two tables: <code>employees</code> and <code>departments</code>. You want to retrieve a list of employees along with their department names. However, some employees may not belong to any department.</p>
<h4><strong>Table: employees</strong></h4>
<table>
<thead>
<tr>
<th>employee_id</th>
<th>employee_name</th>
<th>department_id</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>John Smith</td>
<td>1</td>
</tr>
<tr>
<td>2</td>
<td>Jane Doe</td>
<td>2</td>
</tr>
<tr>
<td>3</td>
<td>Mike Johnson</td>
<td>NULL</td>
</tr>
<tr>
<td>4</td>
<td>Emily White</td>
<td>3</td>
</tr>
</tbody>
</table>
<h4><strong>Table: departments</strong></h4>
<table>
<thead>
<tr>
<th>department_id</th>
<th>department_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>1</td>
<td>IT</td>
</tr>
<tr>
<td>2</td>
<td>HR</td>
</tr>
<tr>
<td>3</td>
<td>Marketing</td>
</tr>
</tbody>
</table>
<h4><strong>LEFT JOIN Query Example:</strong></h4>
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<pre class="language-php"><code>SELECT e.employee_name, d.department_name
FROM employees e
LEFT JOIN departments d
ON e.department_id = d.department_id;
</code></pre>
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<p><strong>Output Example:</strong></p>
<table>
<thead>
<tr>
<th>employee_name</th>
<th>department_name</th>
</tr>
</thead>
<tbody>
<tr>
<td>John Smith</td>
<td>IT</td>
</tr>
<tr>
<td>Jane Doe</td>
<td>HR</td>
</tr>
<tr>
<td>Mike Johnson</td>
<td>NULL</td>
</tr>
<tr>
<td>Emily White</td>
<td>Marketing</td>
</tr>
</tbody>
</table>
<p>In this query:</p>
<ul>
<li><strong><code>e</code></strong> is the alias for the <strong><code>employees</code></strong> table.</li>
<li><strong><code>d</code></strong> is the alias for the <strong><code>departments</code></strong> table.</li>
<li>The <strong><code>LEFT JOIN</code></strong> returns all rows from the <strong><code>employees</code></strong> table, including the employee with no department (Mike Johnson), and for employees without a department, it returns <strong>NULL</strong> for the department name.</li>
</ul>
<hr />
<h3><strong>Explanation of LEFT JOIN</strong></h3>
<ul>
<li><strong>LEFT JOIN</strong> ensures that all rows from the left table (in this case, <strong><code>employees</code></strong>) are returned, regardless of whether a matching row exists in the right table (here, <strong><code>departments</code></strong>).</li>
<li>For employees like <strong>Mike Johnson</strong> who do not have a corresponding department, the department name is returned as <strong>NULL</strong>.</li>
</ul>
<p>This is particularly useful when you want to list all records from one table and only include related records from another table if they exist.</p>
<hr />
<h3><strong>LEFT JOIN with Multiple Tables</strong></h3>
<p>You can also perform a <strong>LEFT JOIN</strong> with multiple tables. For example, if you want to join the <code>employees</code>, <code>departments</code>, and <code>salaries</code> tables, you can do so as follows:</p>
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<pre class="language-php"><code>SELECT e.employee_name, d.department_name, s.salary
FROM employees e
LEFT JOIN departments d ON e.department_id = d.department_id
LEFT JOIN salaries s ON e.employee_id = s.employee_id;
</code></pre>
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<p>This query will return all employees, their department names, and their salaries (if available). Employees without a department or salary will have <strong>NULL</strong> for those columns.</p>
<hr />
<h3><strong>Key Points to Remember</strong></h3>
<ul>
<li><strong>LEFT JOIN</strong> returns all rows from the left table, and matched rows from the right table.</li>
<li>If no matching row is found in the right table, the result will include <strong>NULL</strong> values for columns from the right table.</li>
<li>It is useful when you want to keep all the rows from the left table, even if there is no matching row in the right table.</li>
<li>You can combine multiple <strong>LEFT JOINs</strong> in a query to retrieve data from more than two tables.</li>
</ul>
<hr />
<h3>&nbsp;</h3>
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