{"id":1650,"date":"2018-07-28T18:49:39","date_gmt":"2018-07-29T01:49:39","guid":{"rendered":"https:\/\/sqltutorial.org\/?page_id=1650"},"modified":"2025-02-08T05:00:17","modified_gmt":"2025-02-08T12:00:17","slug":"sql-correlated-subquery","status":"publish","type":"page","link":"https:\/\/www.sqltutorial.org\/sql-correlated-subquery\/","title":{"rendered":"SQL Correlated Subquery"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: In this tutorial, you&#8217;ll learn how to use SQL correlated subquery to perform row-by-row comparisons and complex filtering.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='introduction-to-sql-correlated-subquery'>Introduction to SQL correlated subquery <a href=\"#introduction-to-sql-correlated-subquery\" class=\"anchor\" id=\"introduction-to-sql-correlated-subquery\" title=\"Anchor for Introduction to SQL correlated subquery\">#<\/a><\/h2>\n\n\n\n<p>A correlated <a href=\"https:\/\/www.sqltutorial.org\/sql-subquery\/\">subquery<\/a> is a kind of subquery that depends on the outer query for its input values.<\/p>\n\n\n\n<p>A regular query executes once and provides the result to an outer query. However, a correlated subquery executes repeatedly &#8211; once for each row processed by the outer query.<\/p>\n\n\n\n<p>A correlated subquery has two main characteristics:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Referencing a column from the outer query.<\/li>\n\n\n\n<li>Executing once for each row of the outer query.<\/li>\n<\/ul>\n\n\n\n<p>Typically, you use a correlated subquery in <code><a href=\"https:\/\/www.sqltutorial.org\/sql-where\/\">WHERE<\/a><\/code>, <code><a href=\"https:\/\/www.sqltutorial.org\/sql-select\/\">SELECT<\/a><\/code>, or <code><a href=\"https:\/\/www.sqltutorial.org\/sql-having\/\">HAVING<\/a><\/code> clauses.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='sql-correlated-subquery-example'>SQL correlated subquery example <a href=\"#sql-correlated-subquery-example\" class=\"anchor\" id=\"sql-correlated-subquery-example\" title=\"Anchor for SQL correlated subquery example\">#<\/a><\/h2>\n\n\n\n<p>We&#8217;ll use the <code>employees<\/code> table from the <a href=\"https:\/\/www.sqltutorial.org\/sql-sample-database\/\">HR sample database<\/a>:<\/p>\n\n\n\n<figure class=\"wp-block-image size-full\"><img loading=\"lazy\" decoding=\"async\" width=\"169\" height=\"273\" src=\"https:\/\/www.sqltutorial.org\/wp-content\/uploads\/2016\/03\/employees.png\" alt=\"SQL Correlated Subquery - employees table\" class=\"wp-image-237\"\/><\/figure>\n\n\n\n<p>The following example uses a correlated subquery to find employees whose salary is greater than the average salary of their department:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-1\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">SELECT<\/span>\n  first_name,\n  department_id,\n  salary\n<span class=\"hljs-keyword\">FROM<\/span>\n  employees e1\n<span class=\"hljs-keyword\">WHERE<\/span>\n  salary &gt; (\n    <span class=\"hljs-keyword\">SELECT<\/span>\n      <span class=\"hljs-keyword\">AVG<\/span>(salary) average_salary\n    <span class=\"hljs-keyword\">FROM<\/span>\n      employees e2\n    <span class=\"hljs-keyword\">WHERE<\/span>\n      e2.department_id = e1.department_id\n  )\n<span class=\"hljs-keyword\">ORDER<\/span> <span class=\"hljs-keyword\">BY<\/span>\n  department_id;<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-1\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">SQL (Structured Query Language)<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">sql<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p><a href=\"https:\/\/www.sqltutorial.org\/playground\/?q=U0VMRUNUIGZpcnN0X25hbWUsIGRlcGFydG1lbnRfaWQsIHNhbGFyeSBGUk9NIGVtcGxveWVlcyBlMSBXSEVSRSBzYWxhcnkgPiAoIFNFTEVDVCBBVkcoc2FsYXJ5KSBhdmVyYWdlX3NhbGFyeSBGUk9NIGVtcGxveWVlcyBlMiBXSEVSRSBlMi5kZXBhcnRtZW50X2lkID0gZTEuZGVwYXJ0bWVudF9pZCApIE9SREVSIEJZIGRlcGFydG1lbnRfaWQ7\" target=\"_blank\" rel=\"noreferrer noopener\">Try it<\/a><\/p>\n\n\n\n<p>Output:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-2\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"> first_name | department_id |  salary\n<span class=\"hljs-comment\">------------+---------------+----------<\/span>\n Michael    |             2 | 13000.00\n Den        |             3 | 11000.00\n Shanta     |             5 |  6500.00\n Matthew    |             5 |  8000.00\n Adam       |             5 |  8200.00\n Payam      |             5 |  7900.00\n Bruce      |             6 |  6000.00\n Alexander  |             6 |  9000.00\n John       |             8 | 14000.00\n Karen      |             8 | 13500.00\n Steven     |             9 | 24000.00\n Daniel     |            10 |  9000.00\n Nancy      |            10 | 12000.00\n Shelley    |            11 | 12000.00<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-2\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">SQL (Structured Query Language)<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">sql<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>How the query works.<\/p>\n\n\n\n<p>The outer query selects an employee (<code>e1<\/code>) whose salary is greater than the average salary of all employees in the current employee&#8217;s department.<\/p>\n\n\n\n<p>The subquery calculates the average salary of the current employee&#8217;s department (<code>e2.department_id = e1.department_id<\/code>).<\/p>\n\n\n\n<p>The subquery is executed for each employee selected in the outer query.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='summary'>Summary <a href=\"#summary\" class=\"anchor\" id=\"summary\" title=\"Anchor for Summary\">#<\/a><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>A correlated subquery is a query that uses values from the outer query.<\/li>\n\n\n\n<li>A correlated subquery is executed for each row from the outer query.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id='quiz'>Quiz <a href=\"#quiz\" class=\"anchor\" id=\"quiz\" title=\"Anchor for Quiz\">#<\/a><\/h2>\n\n\n\n<iframe loading=\"lazy\"\n  name=\"quiz\"\n  src=\"\/quiz\/?quiz=correlated-subquery\"\n  height=\"700\"\n  width=\"600\"\n  class=\"iframe\"\n><\/iframe>\n\n\n\n\n\n<h2 class=\"wp-block-heading\" id='databases'>Databases <a href=\"#databases\" class=\"anchor\" id=\"databases\" title=\"Anchor for Databases\">#<\/a><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-correlated-subquery\/\" target=\"_blank\" rel=\"noreferrer noopener\">PostgreSQL Correlated Subquery<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.oracletutorial.com\/oracle-basics\/oracle-correlated-subquery\/\" target=\"_blank\" rel=\"noreferrer noopener\">Oracle Correlated Subquery<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-correlated-subquery\/\" target=\"_blank\" rel=\"noreferrer noopener\">SQL Server Correlated Subquery<\/a><\/li>\n<\/ul>\n<div class=\"helpful-block-content\" data-title=\"\">\n\t<header>\n\t\t<div class=\"wth-question\">Was this tutorial helpful ?<\/div>\n\t\t<div class=\"wth-thumbs\">\n\t\t\t<button\n\t\t\t\tdata-post=\"1650\"\n\t\t\t\tdata-post-url=\"https:\/\/www.sqltutorial.org\/sql-correlated-subquery\/\"\n\t\t\t\tdata-post-title=\"SQL Correlated Subquery\"\n\t\t\t\tdata-response=\"1\"\n\t\t\t\tclass=\"wth-btn-rounded 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