What are SQL JOINs and Their Different Types?

<p><img src="https://img.php.cn/upload/article/000/000/000/173763679712568.jpg" alt="What are SQL JOINs and Their Different Types? "></p> <p><strong>Understanding SQL JOINs and Their Variations</strong></p> <p>SQL JOINs are fundamental for retrieving data from multiple database tables by linking matching columns. Several types of JOINs exist, each offering unique capabilities:</p> <p><strong>The INNER JOIN (or simply JOIN):</strong></p> <p>This retrieves only rows where the join condition is met in <em>both</em> tables. If a row in one table lacks a matching row in the other, it's excluded from the result set.</p> <p><strong>OUTER JOINs: A Broader Perspective</strong></p> <p>OUTER JOINs return all rows from at least one specified table, even if there are no matching rows in the other. Three variations exist:</p> <ul> <li> <strong>LEFT (OUTER) JOIN:</strong> Includes all rows from the left-hand table and matching rows from the right. Non-matching rows from the right table are filled with NULL values.</li> <li> <strong>RIGHT (OUTER) JOIN:</strong> Mirrors the LEFT JOIN, but includes all rows from the right-hand table and matching rows from the left. Non-matching rows from the left are filled with NULLs.</li> <li> <strong>FULL (OUTER) JOIN:</strong> Returns all rows from <em>both</em> tables. If a row in one table has no match in the other, the missing fields are filled with NULLs.</li> </ul> <p><strong>Specialized JOIN Types</strong></p> <ul> <li> <p><strong>NATURAL JOIN:</strong> Automatically joins tables based on identically named columns with compatible data types. It implicitly handles column pairing and eliminates duplicate columns.</p> </li> <li> <p><strong>CROSS JOIN:</strong> Generates a Cartesian product, combining every row from one table with every row from the other. This often results in a large, potentially unwieldy dataset.</p> </li> <li> <p><strong>SELF JOIN:</strong> A table joins with itself, based on a specific condition. This is valuable for identifying relationships within a single table's data.</p> </li> </ul> <p><strong>JOINs Based on Comparison Operators</strong></p> <ul> <li> <p><strong>Equi JOIN:</strong> Uses the equals operator (<code>=</code>) to match rows across tables.</p> </li> <li> <p><strong>Theta JOIN:</strong> Employs a wider range of comparison operators (e.g., <code>></code>, <code><</code>, <code>>=</code>, <code><=</code>, <code>!=</code>) for more flexible row matching.</p> </li> </ul>

The above is the detailed content of What are SQL JOINs and Their Different Types?. For more information, please follow other related articles on the PHP Chinese website!