{"id":20984,"date":"2025-04-01T06:32:08","date_gmt":"2025-04-01T06:32:08","guid":{"rendered":"https:\/\/www.pickl.ai\/blog\/?p=20984"},"modified":"2025-04-01T06:32:09","modified_gmt":"2025-04-01T06:32:09","slug":"sql-subquery","status":"publish","type":"post","link":"https:\/\/www.pickl.ai\/blog\/sql-subquery\/","title":{"rendered":"Understanding SQL Subquery with Examples"},"content":{"rendered":"\n<p>Summary: SQL subqueries simplify complex queries by allowing a query inside another. Learn single-row, multi-row, correlated, and nested subqueries with examples. Optimize performance by using indexed columns and choosing JOINs when applicable. Master SQL subqueries and advance your data science skills with Pickl.AI\u2019s expert-led courses.<\/p>\n\n\n\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_82_2 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Introduction\" >Introduction<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#What_is_SQL_Subquery\" >What is SQL Subquery?<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#How_Does_a_Subquery_Work\" >How Does a Subquery Work?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Basic_Syntax_of_a_Subquery\" >Basic Syntax of a Subquery<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Types_of_SQL_Subqueries\" >Types of SQL Subqueries<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Single-Row_Subqueries\" >Single-Row Subqueries&nbsp;<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Multi-Row_Subqueries\" >Multi-Row Subqueries&nbsp;<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Correlated_Subqueries\" >Correlated Subqueries<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Nested_Subqueries_Multiple_Levels_of_Subqueries\" >Nested Subqueries (Multiple Levels of Subqueries)<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-10\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Subquery_Placement_in_SQL_Queries\" >Subquery Placement in SQL Queries<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-11\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#In_SELECT_Clause_Derived_Values\" >In SELECT Clause (Derived Values)<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-12\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#In_WHERE_Clause_Filtering_Data\" >In WHERE Clause (Filtering Data)<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-13\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#In_FROM_Clause_Using_Subqueries_as_Virtual_Tables\" >In FROM Clause (Using Subqueries as Virtual Tables)<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-14\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#In_HAVING_Clause_Filtering_Aggregated_Data\" >In HAVING Clause (Filtering Aggregated Data)<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-15\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#SQL_Subquery_Examples\" >SQL Subquery Examples<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-16\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Example_1_Finding_the_Highest_Salary_in_a_Company\" >Example 1: Finding the Highest Salary in a Company<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-17\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Example_2_Retrieving_Customers_Who_Placed_the_Largest_Orders\" >Example 2: Retrieving Customers Who Placed the Largest Orders<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-18\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Example_3_Using_Subqueries_in_UPDATE_and_DELETE_Statements\" >Example 3: Using Subqueries in UPDATE and DELETE Statements<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-19\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Performance_Considerations\" >Performance Considerations<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-20\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#When_to_Use_Subqueries_vs_JOINs\" >When to Use Subqueries vs. JOINs<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-21\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Optimising_Subquery_Execution\" >Optimising Subquery Execution<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-22\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Common_Mistakes_to_Avoid\" >Common Mistakes to Avoid<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-23\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Closing_Statements\" >Closing Statements<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-24\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#Frequently_Asked_Questions\" >Frequently Asked Questions<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-25\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#What_is_a_Correlated_Subquery_in_SQL\" >What is a Correlated Subquery in SQL?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-26\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#How_Does_an_SQL_Subquery_Improve_Query_Efficiency\" >How Does an SQL Subquery Improve Query Efficiency?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-27\" href=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/#What_Are_The_Key_Differences_Between_Subqueries_and_JOINs_in_SQL\" >What Are The Key Differences Between Subqueries and JOINs in SQL?<\/a><\/li><\/ul><\/li><\/ul><\/nav><\/div>\n<h2 id=\"introduction\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Introduction\"><\/span><strong>Introduction<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>If you&#8217;ve ever used <a href=\"https:\/\/pickl.ai\/blog\/introduction-to-sql-for-data-science\/\">SQL<\/a> to fetch data, you know how powerful queries can be. But what if you need to run a query inside another query? That\u2019s where SQL subqueries come in! Think of them as little detectives\u2014searching for answers before handing them over to the main query.<\/p>\n\n\n\n<p>In this blog, you\u2019ll learn what an SQL subquery is, how it works, and when to use it. We\u2019ll also break down tricky concepts like correlated subquery SQL with easy-to-follow SQL subquery examples. By the end, you\u2019ll confidently use subqueries like a pro (without breaking a sweat)!&nbsp;<\/p>\n\n\n\n<p><strong>Key Takeaways<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>SQL subqueries allow querying within another query to filter, calculate, or retrieve data dynamically.<\/li>\n\n\n\n<li>Correlated subqueries depend on the main query for execution and run row-by-row, affecting performance.<\/li>\n\n\n\n<li>Types of subqueries include single-row, multi-row, nested, and correlated subqueries for varied use cases.<\/li>\n\n\n\n<li>Use JOINs for faster data retrieval, but subqueries work best for complex filtering.<\/li>\n\n\n\n<li>Optimising SQL subqueries with indexes and efficient query structuring improves performance in large datasets.<\/li>\n<\/ul>\n\n\n\n<h2 id=\"what-is-sql-subquery\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"What_is_SQL_Subquery\"><\/span><strong>What is SQL Subquery?<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>SQL subquery is like a &#8220;query within a query.&#8221; Imagine you are looking for the best pizza place in town, but instead of checking all the restaurants, you first find the top-rated ones and then pick the best among them. That\u2019s exactly how a subquery works in SQL\u2014it helps filter or calculate <a href=\"https:\/\/pickl.ai\/blog\/difference-between-data-and-information\/\">data<\/a> before the main query runs.<\/p>\n\n\n\n<h3 id=\"how-does-a-subquery-work\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"How_Does_a_Subquery_Work\"><\/span><strong>How Does a Subquery Work?<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Think of SQL as a detective solving a case. The main query is the detective, and the subquery is their assistant, who gathers background information first. The assistant (subquery) finds key details, and the detective (main query) uses them to solve the case.<\/p>\n\n\n\n<p>For example, suppose you want to find employees who earn more than the company\u2019s average salary. In that case, a subquery first calculates the average salary, and then the main query selects employees earning above that amount.<\/p>\n\n\n\n<h3 id=\"basic-syntax-of-a-subquery\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Basic_Syntax_of_a_Subquery\"><\/span><strong>Basic Syntax of a Subquery<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A subquery always sits inside parentheses <strong>()<\/strong> and runs before the main query. Here\u2019s how it looks:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXdU6U8hnJLL3wHVvFbBuo7aJYfr1z9G4DELgXO1oGlBBrkZ27ZNKtWb9S1gtt-juhQ8s4X-c9ie90rWYD_TFOtT3oosLDBAnP6j5ecqmmePBgWuYV0LWxqc2zUUbLcAyPz5N2nrOA?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"SQL subquery to find employees earning above average.\"\/><\/figure>\n\n\n\n<p>This finds employees who earn more than the average salary. A correlated subquery SQL works differently\u2014it depends on each row of the main query while running. We\u2019ll explore that soon!<\/p>\n\n\n\n<h2 id=\"types-of-sql-subqueries\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Types_of_SQL_Subqueries\"><\/span><strong>Types of SQL Subqueries<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>SQL subqueries come in different types, each serving a unique purpose. A subquery is a query inside another query, helping to fetch specific data. Understanding these types will help you write better and more efficient SQL queries.<\/p>\n\n\n\n<h3 id=\"single-row-subqueries\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Single-Row_Subqueries\"><\/span><strong>Single-Row Subqueries&nbsp;<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A single-row subquery returns only one value. This means the result is a single number, text, or date. It is often used with comparison operators like <strong>=, &gt;, &lt;, &gt;=, and &lt;=<\/strong>.<\/p>\n\n\n\n<p><strong>Example:<\/strong> Suppose you want to find the employee with the highest salary. You can first get the highest salary using a subquery and then use it in the main query:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXdmW3SP22MVj0CXBANRaQb_hMHpCn5lq0G132rEiIvfhVdy8l-Klw_h4OW2bJ0_NytZtwYqAoNyOxKkXm-9wyVbTDfHWv1eDDDiJn4BBPuETMFt9OAcTwuQHX19sdGqLeqckai3?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Query to find the highest-paid employee.\"\/><\/figure>\n\n\n\n<h3 id=\"multi-row-subqueries\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Multi-Row_Subqueries\"><\/span><strong>Multi-Row Subqueries&nbsp;<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A multi-row subquery returns more than one value. It is used when you need a list of results instead of just one. Operators like <strong>IN, ANY, and ALL<\/strong> are used in these subqueries.<\/p>\n\n\n\n<p><strong>Example:<\/strong> Find employees who work in the same department as &#8220;John&#8221;:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXfwl7-TQoq8NJQsQdusU4TAttsQJLZjGxtc3q5BGvwnMZv0UDYmXj9WWsB74Izn2NHh-eoo6_QQB499i9cefr3LD9jjTHD75EdrbMlBUhRi9ulI3UZSoCTpUI969SEi6O18e62W?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Query to find employees in John's department.\"\/><\/figure>\n\n\n\n<h3 id=\"correlated-subqueries\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Correlated_Subqueries\"><\/span><strong>Correlated Subqueries<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A correlated subquery depends on the main query for its execution. It runs once for each row processed in the main query, making it slower than other types.<\/p>\n\n\n\n<p><strong>Example:<\/strong> Find employees earning more than the average salary of their department:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXe3ByuTgPlFrgI0rE7IIy8PWaHmwmSxUMpqDbX5kZWfb2m7yfXItno911QuEbNYoh7rTSn8bdnk3ELOg-rG8sFNcZmwgI_DT7W1uVXbyhaaVKk1fFunr9L3QQuGKpDeVEvXqeapJA?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Query to find employees earning above dept. average \"\/><\/figure>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXdYLU2PDW0oOJz6i2uGmoBak9xI05ZfbD5B8YaSjvLM8F7pSPm-YQN6joV3JLQ3eVy4JXYwVDbSs2e1QlPFGHeI41n4zL_dhQjCfzZclNjMbWPGLh8BOfe65PtfWReMXY8fQPFx?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Query to find employees earning above dept. average \"\/><\/figure>\n\n\n\n<h3 id=\"nested-subqueries-multiple-levels-of-subqueries\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Nested_Subqueries_Multiple_Levels_of_Subqueries\"><\/span><strong>Nested Subqueries (Multiple Levels of Subqueries)<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A nested subquery contains another subquery inside it. SQL processes these from the innermost query to the outermost.<\/p>\n\n\n\n<p><strong>Example:<\/strong> Find the names of employees working in the department with the highest number of employees:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXe4BPBehubn-nEJHn1BMKWsB42AAft2GEIUm2vAqxH-3QJaewGPOgLh0wztNwXHBydxWy8fWl7WSyVnCJbE7Hnay6Lz22pF-THR9gRH0qZv2RC0lzG6qVAmP0PI-ArKN_-Nbe0Srw?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Query to find employees in the largest department \"\/><\/figure>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXcGcGcBS7cWKOWpeSS-cIJSVs4W_xqvs9uZsUu5WR1AE1bumZ1sxJvqKrAi0L9avMm5xllpJwYYAXdke3i7gMIdhBkg3RyQDfmAlXln9O9qpsufl3_SDHYNJqUfodKTYo62M6rL?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Query to find employees in the largest department \"\/><\/figure>\n\n\n\n<h2 id=\"subquery-placement-in-sql-queries\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Subquery_Placement_in_SQL_Queries\"><\/span><strong>Subquery Placement in SQL Queries<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>A subquery is a smaller query inside a bigger query. It helps <a href=\"https:\/\/pickl.ai\/blog\/data-collection-methods-techniques\/\">fetch data<\/a> in a structured way by breaking a complex question into smaller parts. Subqueries can be placed in different parts of an SQL statement to achieve various tasks. Below are the key places where you can use subqueries in SQL.<\/p>\n\n\n\n<h3 id=\"in-select-clause-derived-values\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"In_SELECT_Clause_Derived_Values\"><\/span><strong>In SELECT Clause (Derived Values)<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Sometimes, you need to calculate a value before displaying it in the result. A subquery in the SELECT clause helps generate these values dynamically.<\/p>\n\n\n\n<p>For example, imagine you have a list of employees and want to show each person\u2019s salary along with the average salary of all employees. You can use a subquery to get the average salary and display it alongside individual salaries.<\/p>\n\n\n\n<p><strong>Example:<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXeCr4L4Ws_-VO8YhnK7Ds68vtUIAZSyIW9ofcNVLvkCMJTmj8KUUDd4863CLwUjUvCYwdYR2IJEknQYwXfnNZZViTRrTw3254MfEkdCP3ZwP6ROBCsbBlb4SN8DKp99nq7mLJiUsw?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Show salary with average salary in SQL.\"\/><\/figure>\n\n\n\n<h3 id=\"in-where-clause-filtering-data\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"In_WHERE_Clause_Filtering_Data\"><\/span><strong>In WHERE Clause (Filtering Data)<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A subquery in the <strong>WHERE clause<\/strong> helps filter data based on a condition. It is useful when you need to compare values with another dataset.<\/p>\n\n\n\n<p>For example, if you want to find employees who earn <strong>more than the average salary<\/strong>, you can use a subquery in the WHERE clause.<\/p>\n\n\n\n<p><strong>Example:<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXfdsRQ5PDreQgi82Khfd2QdCx1vVAWeUFri1yDJ_T3HCMdVEHVNaNrxrC9zR05KFNTsaNDCsPT24oLC8hlELbZve4u-cZj83wBoyz4qqNCH3mbvGOulKB8POf8e9K14XrjpmjwLUA?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Find employees earning above the average salary.\"\/><\/figure>\n\n\n\n<h3 id=\"in-from-clause-using-subqueries-as-virtual-tables\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"In_FROM_Clause_Using_Subqueries_as_Virtual_Tables\"><\/span><strong>In FROM Clause (Using Subqueries as Virtual Tables)<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Sometimes, you may need to use the result of a subquery as a <strong>temporary table<\/strong>. This is useful when breaking down complex queries into smaller, readable parts.<\/p>\n\n\n\n<p>For example, let\u2019s say you want to see the <strong>top 3 highest-paid employees<\/strong>. You can first create a temporary table using a subquery, then select from it.<\/p>\n\n\n\n<p><strong>Example:<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXcso3cRBsZA-F490-SP7cFr9seSgfxv---X9Tt8eyKmNgWNkjga4nuQ1QvBJ5a1mStgfyH2Y_pjuep0uMaDqVrHhPzJOvfakQWzkgubRbq3-hQLcH6Egllp-Ie0z75ufHxUKC8VVA?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Get the top 3 highest-paid employees in SQL.\"\/><\/figure>\n\n\n\n<h3 id=\"in-having-clause-filtering-aggregated-data\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"In_HAVING_Clause_Filtering_Aggregated_Data\"><\/span><strong>In HAVING Clause (Filtering Aggregated Data)<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A subquery in the HAVING clause is useful when filtering grouped data. It is mainly used with aggregate functions like SUM, COUNT, AVG, etc.<\/p>\n\n\n\n<p>For example, suppose you want to list departments with higher total salary expenses than the average department salary expense.<\/p>\n\n\n\n<p><strong>Example:<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXdwJSXoTB4oPE5yVqq2VVUbOSwfhQNp4TZDls6To5CfvAq1b_ofY74c29w_rBTaHxyJCj1usnrZsIsJl9hBrLCXschMBxx4ct2PimRXgoCU_XBDJBsYubCVR2fMsxbsvWhEuIKDQg?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"Find departments with above-average salary expenses.\"\/><\/figure>\n\n\n\n<h2 id=\"sql-subquery-examples\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"SQL_Subquery_Examples\"><\/span><strong>SQL Subquery Examples<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>Subqueries help us find specific information from a database by using one query inside another. Let\u2019s look at some real-world examples to understand how subqueries work.<\/p>\n\n\n\n<h3 id=\"example-1-finding-the-highest-salary-in-a-company\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Example_1_Finding_the_Highest_Salary_in_a_Company\"><\/span><strong>Example 1: Finding the Highest Salary in a Company<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Imagine a company that wants to find the employee with the highest salary. Instead of manually checking each salary, we can use a subquery to find the maximum salary and then retrieve the employee\u2019s details.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXf6LCBGPYaPERUOTB-F6XSQKU4C_TvPEqfxZO-fLph-ztqWBN_p9JovwP9jxMOB5CJxJbx9hjK_5Q1a6oIBEI9yXU-XevnT-m4byQRLuKNB70PO8geDqN0IF9NdHCB1KFG40iWbHg?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"SQL query to find the highest-paid employee.\"\/><\/figure>\n\n\n\n<h3 id=\"example-2-retrieving-customers-who-placed-the-largest-orders\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Example_2_Retrieving_Customers_Who_Placed_the_Largest_Orders\"><\/span><strong>Example 2: Retrieving Customers Who Placed the Largest Orders<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A business wants to find customers who place the largest order. We can use a subquery to get the highest order amount and then find the customers who spent that much.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXfml_i1NdG_pLaJO27MIeENRcS_9KWvqpnxbf0T6yb3MIUQjlxpISTu3psfAGmZcGmtpd8DNNfcZ8bvbTUY0l79CF3oCsKsFnYamlDb2C9DUyaiQSvyasP6q3ZlggWA9nWXhnXB?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"SQL query to get top-spending customers.\"\/><\/figure>\n\n\n\n<h3 id=\"example-3-using-subqueries-in-update-and-delete-statements\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Example_3_Using_Subqueries_in_UPDATE_and_DELETE_Statements\"><\/span><strong>Example 3: Using Subqueries in UPDATE and DELETE Statements<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Subqueries also help update or delete records efficiently. Suppose we need to increase the salary of employees who earn less than the average salary:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXd2ulAVmKWx1Rn2AsyMm1jM-c2MikNR2Pt_3jFcmZpz3KrMjLO-nBa7y77sutRdyxzEIsslCPHPzOg3bsEmBRLUeoiimXax3uERsouOp9Lrb4c0xcqcmxEb02x2mBqys9po8uHaMg?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"SQL query to update low salaries by 10%.\"\/><\/figure>\n\n\n\n<p>Similarly, if we want to delete inactive customers who never placed an order:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img decoding=\"async\" src=\"https:\/\/lh7-rt.googleusercontent.com\/docsz\/AD_4nXfGPgWxYhWa2aNmU_BgH4JAp2I4RrFXxjvpGeWIeDTs11B03Zr37illcTsO4MtTG9bMSdcJ02-NzYkZWnIJp33z6f-a3L1JcLEuMx9zYqsKYDdrYhBPk84mDWYsbwIV57tse3Mv?key=XQgv_CmtnR-W8pCgqb-AOUeU\" alt=\"SQL query to remove inactive customers.\"\/><\/figure>\n\n\n\n<h2 id=\"performance-considerations\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Performance_Considerations\"><\/span><strong>Performance Considerations<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>Using subqueries in SQL can make queries more powerful, but they can also slow down performance if not used correctly. Understanding when to use subqueries, how to optimise them, and common mistakes to avoid will help write efficient SQL queries.<\/p>\n\n\n\n<h3 id=\"when-to-use-subqueries-vs-joins\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"When_to_Use_Subqueries_vs_JOINs\"><\/span><strong>When to Use Subqueries vs. JOINs<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Subqueries work well when you need to break a problem into smaller parts. They are useful for dynamically filtering data, performing calculations, or retrieving values. However, subqueries can sometimes make queries run slower because the database processes them separately for each row.<\/p>\n\n\n\n<p>JOINs are often a better choice when combining data from multiple tables. Unlike subqueries, JOINs usually perform better because they process all related data simultaneously. If the same result can be achieved using a JOIN instead of a subquery, it&#8217;s usually the faster option.<\/p>\n\n\n\n<h3 id=\"optimising-subquery-execution\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Optimising_Subquery_Execution\"><\/span><strong>Optimising Subquery Execution<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>To speed up subqueries, use indexed columns whenever possible. Indexing helps the <a href=\"https:\/\/pickl.ai\/blog\/database-vs-data-warehouse\/\">database<\/a> find data faster. Also, avoid unnecessary subqueries\u2014sometimes, rewriting a query using a JOIN or a <a href=\"https:\/\/learn.microsoft.com\/en-us\/sql\/t-sql\/queries\/with-common-table-expression-transact-sql?view=sql-server-ver16\" rel=\"nofollow\">common table expression<\/a> (CTE) can improve performance.<\/p>\n\n\n\n<h3 id=\"common-mistakes-to-avoid\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Common_Mistakes_to_Avoid\"><\/span><strong>Common Mistakes to Avoid<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>One common mistake is using subqueries inside loops, which can slow down execution. Another is selecting too many columns when only one is needed. Also, avoid correlated subqueries when a simple query can do the job more efficiently.<\/p>\n\n\n\n<h2 id=\"closing-statements\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Closing_Statements\"><\/span><strong>Closing Statements<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<p>SQL subqueries are powerful tools for simplifying complex queries and enhancing data retrieval. You can write more efficient SQL queries by mastering single-row, multi-row, correlated, and nested subqueries.&nbsp;<\/p>\n\n\n\n<p>Understanding when to use subqueries versus JOINs ensures better performance. Whether filtering data or performing calculations, subqueries streamline SQL operations. You can strengthen your SQL skills and advance in <a href=\"https:\/\/pickl.ai\/blog\/what-is-data-science-comprehensive-guide\/\">data science<\/a> by enrolling in <a href=\"https:\/\/www.pickl.ai\/\">Pickl.AI\u2019s data science courses<\/a>.&nbsp;<\/p>\n\n\n\n<p>These courses cover SQL, <a href=\"https:\/\/pickl.ai\/blog\/what-is-machine-learning\/\">machine learning<\/a>, and other essential techniques for aspiring <a href=\"https:\/\/pickl.ai\/blog\/data-analyst-vs-data-scientist\/\">data analysts and scientists<\/a>. Start learning today to gain hands-on expertise and excel in your data-driven career!<\/p>\n\n\n\n<h2 id=\"frequently-asked-questions\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Frequently_Asked_Questions\"><\/span><strong>Frequently Asked Questions<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h2>\n\n\n\n<h3 id=\"what-is-a-correlated-subquery-in-sql\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"What_is_a_Correlated_Subquery_in_SQL\"><\/span><strong>What is a Correlated Subquery in SQL?<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>A correlated subquery is an SQL query that references columns from the main query and executes once for each row processed. Unlike standard subqueries, it depends on the outer query for execution, making it less efficient. It\u2019s commonly used for comparisons within grouped datasets.<\/p>\n\n\n\n<h3 id=\"how-does-an-sql-subquery-improve-query-efficiency\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"How_Does_an_SQL_Subquery_Improve_Query_Efficiency\"><\/span><strong>How Does an SQL Subquery Improve Query Efficiency?<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>An SQL subquery improves efficiency by breaking down complex queries into smaller, manageable parts. It helps dynamically filter, calculate, and retrieve data before executing the main query. However, excessive use of subqueries may slow performance, so optimising with indexes and using JOINs when applicable is recommended.<\/p>\n\n\n\n<h3 id=\"what-are-the-key-differences-between-subqueries-and-joins-in-sql\" class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"What_Are_The_Key_Differences_Between_Subqueries_and_JOINs_in_SQL\"><\/span><strong>What Are The Key Differences Between Subqueries and JOINs in SQL?<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n\n\n\n<p>Subqueries filter or retrieve values dynamically within a single query, whereas JOINs combine data from multiple tables in a structured way. JOINs often perform better for retrieving related data in bulk, while subqueries are useful for conditional filtering or calculating values before query execution.<\/p>\n","protected":false},"excerpt":{"rendered":"Master SQL subqueries, correlated subquery SQL, and more with practical SQL subquery examples and best practices.\n","protected":false},"author":4,"featured_media":20985,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[613],"tags":[3885],"ppma_author":[2169,2185],"class_list":{"0":"post-20984","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-sql","8":"tag-sql-subquery"},"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v20.3 (Yoast SEO v27.3) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>SQL Subquery with Examples<\/title>\n<meta name=\"description\" content=\"Learn SQL subqueries, including correlated subquery SQL, with examples. 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Master SQL filtering, and optimisations for efficient data retrieval.\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.pickl.ai\/blog\/sql-subquery\/\" \/>\n<meta property=\"og:site_name\" content=\"Pickl.AI\" \/>\n<meta property=\"article:published_time\" content=\"2025-04-01T06:32:08+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2025-04-01T06:32:09+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/www.pickl.ai\/blog\/wp-content\/uploads\/2025\/04\/image11.png\" \/>\n\t<meta property=\"og:image:width\" content=\"800\" \/>\n\t<meta property=\"og:image:height\" content=\"500\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"author\" content=\"Neha Singh, Ajay Goyal\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Neha Singh\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"11 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\\\/\\\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/#article\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/\"},\"author\":{\"name\":\"Neha Singh\",\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/#\\\/schema\\\/person\\\/2ad633a6bc1b93bc13591b60895be308\"},\"headline\":\"Understanding SQL Subquery with Examples\",\"datePublished\":\"2025-04-01T06:32:08+00:00\",\"dateModified\":\"2025-04-01T06:32:09+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/\"},\"wordCount\":1601,\"commentCount\":0,\"image\":{\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/wp-content\\\/uploads\\\/2025\\\/04\\\/image11.png\",\"keywords\":[\"SQL Subquery\"],\"articleSection\":[\"SQL\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/\",\"url\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/\",\"name\":\"SQL Subquery with Examples\",\"isPartOf\":{\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/#website\"},\"primaryImageOfPage\":{\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/#primaryimage\"},\"image\":{\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/sql-subquery\\\/#primaryimage\"},\"thumbnailUrl\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/wp-content\\\/uploads\\\/2025\\\/04\\\/image11.png\",\"datePublished\":\"2025-04-01T06:32:08+00:00\",\"dateModified\":\"2025-04-01T06:32:09+00:00\",\"author\":{\"@id\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/#\\\/schema\\\/person\\\/2ad633a6bc1b93bc13591b60895be308\"},\"description\":\"Learn SQL subqueries, including correlated subquery SQL, with examples. 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The 8 years long journey as a content writer and editor has made me relaize the significance and power of choosing the right words. Prior to my writing journey, I was a trainer and human resource manager. WIth more than a decade long professional journey, I find myself more powerful as a wordsmith. As an avid writer, everything around me inspires me and pushes me to string words and ideas to create unique content; and when I\u2019m not writing and editing, I enjoy experimenting with my culinary skills, reading, gardening, and spending time with my adorable little mutt Neel.\",\"url\":\"https:\\\/\\\/www.pickl.ai\\\/blog\\\/author\\\/nehasingh\\\/\"}]}<\/script>\n<!-- \/ Yoast SEO Premium plugin. -->","yoast_head_json":{"title":"SQL Subquery with Examples","description":"Learn SQL subqueries, including correlated subquery SQL, with examples. 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