<p>The key to writing a good SQL subquery is to clarify its purpose, master the result type and optimize performance. 1. Make it clear that subqueries are to generate intermediate results for external query use, not universal tools, multi-layer nesting should be avoided, JOIN should be used first and alias or CTE should be improved readability; 2. Pay attention to the type of result returned by subqueries, single value is used with comparison operators, one row and multiple columns are directly compared or wrapped in multiple fields, one column and multiple rows are used with IN, EXISTS, etc., and multiple columns and multiple rows are used as temporary tables; 3. In terms of performance optimization, avoid repeated calculations caused by related subqueries, and can improve efficiency by rewriting it to JOIN group statistics, pre-calculate the subquery results or establishing indexes. </p> <p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175225555310517.jpeg" class="lazy" alt="Writing Efficient SQL Subqueries for Complex Data Retrieval"></p> <p> The key to writing SQL subqueries well is to understand its role and applicable scenarios. Many people use subqueries when processing complex data retrieval, but if you don't pay attention, it is easy to write inefficient and logically chaotic statements. In fact, as long as you master a few key points, you can write efficient and clear subqueries. </p> <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175225555461046.jpeg" class="lazy" alt="Writing Efficient SQL Subqueries for Complex Data Retrieval"><hr> <h3 id="Clarify-the-purpose-of-subquery-it-is-an-intermediate-result-not-a-universal-tool"> 1. Clarify the purpose of subquery: it is an intermediate result, not a universal tool</h3> <p> Subqueries are essentially used to generate a temporary result set for use by outer query. It is not a "pigpi" that solves all problems, and should only be used if it really needs to rely on internal query results. </p> <img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175225555544670.jpeg" class="lazy" alt="Writing Efficient SQL Subqueries for Complex Data Retrieval"><p> For example, if you want to find products with sales higher than average sales:</p><pre class='brush:php;toolbar:false;'> SELECT product_name, sales FROM products WHERE sales > (SELECT AVG(sales) FROM products);</pre><p> Here, the subquery <code>(SELECT AVG(sales) FROM products)</code> provides a reference value so that the outer query can filter out the data that meets the criteria. </p><img src="/static/imghw/default1.png" data-src="https://img.php.cn/upload/article/000/000/000/175225555697352.jpeg" class="lazy" alt="Writing Efficient SQL Subqueries for Complex Data Retrieval" /><p> <strong>suggestion:</strong></p><ul><li> Try to avoid multi-layer nesting, as it will be difficult to maintain above three layers.</li><li> If it can be implemented <code>JOIN</code> , give priority to <code>JOIN</code> , which usually has better performance</li><li> Improve readability using alias or CTE (common table expression)</li></ul><hr /><h3 id="Pay-attention-to-the-type-of-result-returned-by-the-subquery-single-value-row-column-or-table"> 2. Pay attention to the type of result returned by the subquery: single value, row, column or table?</h3><p> Different subqueries return different types of results, and the corresponding operators are also different:</p><ul><li> <strong>Single value</strong> : Use comparison operators ( <code>></code> , <code><</code> , <code>=</code> etc.)</li><li> <strong>Multiple columns in one row</strong> : use <code>ROW()</code> or directly compare multiple fields</li><li> <strong>Multiple rows in one column</strong> : use <code>IN</code> , <code>NOT IN</code> , <code>EXISTS</code> , <code>ANY/SOME</code> , <code>ALL</code></li><li> <strong>Multiple columns and multiple rows</strong> : usually used as a temporary table, and needs to be combined with <code>FROM</code> clause</li></ul><p> For example, if you want to find customers with order records:</p><pre class='brush:php;toolbar:false;'> SELECT customer_id, name FROM customers WHERE customer_id IN (SELECT customer_id FROM orders);</pre><p> The subquery here returns a list of customer IDs, so it is appropriate to use <code>IN</code> .</p><p> <strong>Common misunderstandings:</strong></p><ul><li> When using <code>NOT IN</code> , be careful whether <code>NULL</code> value is included, otherwise the result may be empty</li><li> <code>EXISTS</code> is more efficient than <code>IN</code> , especially when there are large data volumes, because it stops looking for once it finds a match.</li></ul><hr /><h3 id="Performance-optimization-Don-t-let-subquery-slow-down-your-query-speed"> 3. Performance optimization: Don't let subquery slow down your query speed</h3><p> Subqueries can easily become a performance bottleneck, especially without index support. Many databases will rerun subqueries every time an outer query is executed, causing duplicate calculations.</p><p> For example, the following example may be slow:</p><pre class='brush:php;toolbar:false;'> SELECT name FROM employees e WHERE salary > (SELECT AVG(salary) FROM employees WHERE department = e.department);</pre><p> This is a related subquery because the subquery refers to the field <code>e.department</code> of the outer query, which means that each employee record has to run the subquery once individually.</p><p> <strong>Optimization method:</strong></p><ul><li> Change to <code>JOIN</code> group statistics method</li><li> Extract the subquery first and then make associations</li><li> Create appropriate indexes on the fields involved</li></ul><p> For example, rewrite the above example:</p><pre class='brush:php;toolbar:false;'> SELECT e.name FROM employees e JOIN ( SELECT department, AVG(salary) AS avg_salary FROM employees GROUP BY department ) dept_avg ON e.department = dept_avg.department WHERE e.salary > dept_avg.avg_salary;</pre><p> In this way, the average salary of each department is calculated first and then compared, which is more efficient.</p> <hr> <p> Basically that's it. Subquery itself is not difficult, but to use it well, you have to use it in combination with the specific scenario to determine whether it is nested or split, whether it is <code>IN</code> or <code>EXISTS</code> , whether it is indexed or not, etc. If you don't pay attention to these details, you may write a slow query.</p>

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