How Does SQL's `(col1, col2) < (val1, val2)` Row Value Comparison Work?

<p><img src="https://img.php.cn/upload/article/000/000/000/173646916331017.jpg" alt="How Does SQL's `(col1, col2) < (val1, val2)` Row Value Comparison Work? "></p> <p><strong>SQL Row Value Comparison: Understanding <code>(col1, col2) < (val1, val2)</code> Syntax</strong></p> <p>SQL's <code>WHERE</code> clause allows flexible and diverse data selection conditions, including comparing multiple columns in a single expression. A common syntax is <code>WHERE (col1, col2) < (val1, val2)</code>. </p> <p>This type of condition is called "row value comparison" or "row-by-row comparison". It uses the row constructor (denoted ROW(col1, col2)) to compare the value of an entire row with the value of another row. The operators <code><</code>, <code>></code>, <code><=</code>, <code>>=</code>, <code>=</code> and <code>!=</code> all support this comparison. </strong></p> <p>The functionality of row value comparison dates back to the SQL-92 standard, and PostgreSQL is currently the only major relational database management system (RDBMS) to fully support it, including index optimization. </p> <p>In the example provided, the meaning of <code>WHERE (col1, col2) < (val1, val2)</code> is: if <code>col1</code> is less than <code>val1</code>, or if <code>col1</code> is equal to <code>val1</code> and <code>col2</code> is less than <code>val2</code>, then the row satisfies the condition. </p> <div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"><code class="language-sql">--示例 SELECT * FROM my_table WHERE (col1, col2) < (10, 20);</code></pre><div class="contentsignin">Copy after login</div></div> <p>Note that this is a different syntax than <code>WHERE col1 < val1 AND col2 < val2</code>. The latter requires that both <code>col1</code> and <code>col2</code> must be smaller than their corresponding values, while row value comparisons are compared row by row. </p> <p>To perform row value comparisons efficiently, PostgreSQL uses multi-column indexes, and the index order can be <code>(col1, col2)</code> or <code>(col1 DESC, col2 DESC)</code>. Indexes with mixed sort directions (e.g. <code>(col1 ASC, col2 DESC)</code>) do not apply. </p> <p>By understanding the concept of row value comparison, developers can leverage its capabilities to perform data selection tasks efficiently, especially in keyset pagination scenarios. </p>

The above is the detailed content of How Does SQL's `(col1, col2) < (val1, val2)` Row Value Comparison Work?. For more information, please follow other related articles on the PHP Chinese website!