{"id":264,"date":"2016-03-26T04:05:07","date_gmt":"2016-03-26T04:05:07","guid":{"rendered":"https:\/\/sqltutorial.org\/?page_id=264"},"modified":"2025-02-07T21:12:32","modified_gmt":"2025-02-08T04:12:32","slug":"sql-self-join","status":"publish","type":"page","link":"https:\/\/www.sqltutorial.org\/sql-self-join\/","title":{"rendered":"SQL SELF JOIN"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you&#8217;ll how to compare rows within the same table using the SQL self-join technique.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='introduction-to-sql-self-join'>Introduction to SQL self-join <a href=\"#introduction-to-sql-self-join\" class=\"anchor\" id=\"introduction-to-sql-self-join\" title=\"Anchor for Introduction to SQL self-join\">#<\/a><\/h2>\n\n\n\n<p>Typically, you use a join such as <a href=\"https:\/\/www.sqltutorial.org\/sql-inner-join\/\">inner join<\/a>, <a href=\"https:\/\/www.sqltutorial.org\/sql-left-join\/\">left join<\/a>, and <a href=\"https:\/\/www.sqltutorial.org\/sql-right-join\/\">right join<\/a> to merge rows from two tables based on a condition.<\/p>\n\n\n\n<p>However, a join doesn&#8217;t have to involve multiple tables. You can use a join to compare rows within the same table. In this case, you join a table to itself that forms a self-join.<\/p>\n\n\n\n<p>A self-join is a join that compares the rows within the same table. A self-join uses an inner join, left join, or right join that joins a table to itself. It uses table aliases to treat the same table as separate tables within the same query.<\/p>\n\n\n\n<p>Here&#8217;s the basic syntax of a self-join:<\/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  select_list\n<span class=\"hljs-keyword\">FROM<\/span>\n  table1 t1\n  <span class=\"hljs-keyword\">INNER<\/span> <span class=\"hljs-keyword\">JOIN<\/span> table1 <span class=\"hljs-keyword\">AS<\/span> t2 <span class=\"hljs-keyword\">ON<\/span> t1.column1 = t2.column2;<\/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>In this syntax, you can use the <code>LEFT JOIN<\/code>, <code>RIGHT JOIN<\/code>, and <code>FULL JOIN<\/code> instead of the <code>INNER JOIN<\/code>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='sql-self-join-example'>SQL self-join example <a href=\"#sql-self-join-example\" class=\"anchor\" id=\"sql-self-join-example\" title=\"Anchor for SQL self-join 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\"><img loading=\"lazy\" decoding=\"async\" width=\"169\" height=\"273\" src=\"https:\/\/www.sqltutorial.org\/wp-content\/uploads\/2016\/03\/employees.png\" alt=\"SQL SELF JOIN - employees table.\" class=\"wp-image-237\"\/><\/figure>\n\n\n\n<p>In the <code>employees<\/code> table:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>The <code>employee_id<\/code> serves as a unique identifier for each employee.<\/li>\n\n\n\n<li>The <code>manager_id<\/code> represents the <code>employee_id<\/code> of the manager to whom the current employee reports. If the <code>manager_id<\/code> is <code>NULL<\/code>, it means the employee is the CEO, without a manager.<\/li>\n<\/ul>\n\n\n\n<p>The following query uses an inner join to join the <code>employees<\/code> table to itself to get the information of who reports to whom:<\/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\"><span class=\"hljs-keyword\">SELECT<\/span>\n  e.first_name employee,\n  m.first_name manager\n<span class=\"hljs-keyword\">FROM<\/span>\n  employees e\n  <span class=\"hljs-keyword\">LEFT<\/span> <span class=\"hljs-keyword\">JOIN<\/span> employees m <span class=\"hljs-keyword\">ON<\/span> m.employee_id = e.manager_id\n<span class=\"hljs-keyword\">ORDER<\/span> <span class=\"hljs-keyword\">BY<\/span>\n  manager <span class=\"hljs-keyword\">NULLS<\/span> <span class=\"hljs-keyword\">FIRST<\/span>;<\/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><a href=\"https:\/\/www.sqltutorial.org\/playground\/?q=U0VMRUNUIGUuZmlyc3RfbmFtZSBlbXBsb3llZSwgbS5maXJzdF9uYW1lIG1hbmFnZXIgRlJPTSBlbXBsb3llZXMgZSBJTk5FUiBKT0lOIGVtcGxveWVlcyBtIE9OIG0uZW1wbG95ZWVfaWQgPSBlLm1hbmFnZXJfaWQgT1JERVIgQlkgbWFuYWdlcjs%3D\" 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-3\" data-shcb-language-name=\"plaintext\" data-shcb-language-slug=\"plaintext\"><span><code class=\"hljs language-plaintext\">  employee   |  manager\n-------------+-----------\n Valli       | Alexander\n Diana       | Alexander\n Bruce       | Alexander\n David       | Alexander\n Guy         | Den\n Karen       | Den\n Alexander   | Den\n Shelli      | Den\n Sigal       | Den\n Alexander   | Lex\n Irene       | Matthew\n...<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-3\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">plaintext<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">plaintext<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\n\n\n<p>Since the inner join clause only includes the rows with matching rows in the other table, the query does not include the CEO in the result set of the query.<\/p>\n\n\n\n<p>To include the CEO in the result set, you can use a <code>LEFT JOIN<\/code> clause instead of the <code>INNER JOIN<\/code> clause.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='performing-a-self-join-using-a-left-join-clause'>Performing a self-join using a LEFT JOIN clause <a href=\"#performing-a-self-join-using-a-left-join-clause\" class=\"anchor\" id=\"performing-a-self-join-using-a-left-join-clause\" title=\"Anchor for Performing a self-join using a LEFT JOIN clause\">#<\/a><\/h2>\n\n\n\n<p>The following statement performs a self-join using a <code>LEFT JOIN<\/code> clause:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-4\" 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  e.first_name employee,\n  m.first_name manager\n<span class=\"hljs-keyword\">FROM<\/span>\n  employees e\n  <span class=\"hljs-keyword\">LEFT<\/span> <span class=\"hljs-keyword\">JOIN<\/span> employees m <span class=\"hljs-keyword\">ON<\/span> m.employee_id = e.manager_id\n<span class=\"hljs-keyword\">ORDER<\/span> <span class=\"hljs-keyword\">BY<\/span>\n  manager <span class=\"hljs-keyword\">NULLS<\/span> <span class=\"hljs-keyword\">FIRST<\/span>;<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-4\"><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=U0VMRUNUIGUuZmlyc3RfbmFtZSB8fCAnICcgfHwgZS5sYXN0X25hbWUgQVMgZW1wbG95ZWUsIG0uZmlyc3RfbmFtZSB8fCAnICcgfHwgbS5sYXN0X25hbWUgQVMgbWFuYWdlciBGUk9NIGVtcGxveWVlcyBlIExFRlQgSk9JTiBlbXBsb3llZXMgbSBPTiBtLmVtcGxveWVlX2lkID0gZS5tYW5hZ2VyX2lkIE9SREVSIEJZIG1hbmFnZXI7\" 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-5\" data-shcb-language-name=\"plaintext\" data-shcb-language-slug=\"plaintext\"><span><code class=\"hljs language-plaintext\">  employee   |  manager\n-------------+-----------\n Steven      | NULL\n Bruce       | Alexander\n David       | Alexander\n Valli       | Alexander\n Diana       | Alexander\n Sigal       | Den\n Guy         | Den\n Alexander   | Den\n Shelli      | Den\n Karen       | Den\n Alexander   | Lex\n Irene       | Matthew\n...<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-5\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">plaintext<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">plaintext<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\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 self-join is a join that compares rows within the same tables.<\/li>\n\n\n\n<li>Use an inner join, left join, or full join to perform a self-join.<\/li>\n\n\n\n<li>Use table aliases in the query to treat the same table as two separate tables.<\/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=self-join\"\n  height=\"700\"\n  width=\"600\"\n  class=\"iframe\"\n><\/iframe>\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-self-join\/\" target=\"_blank\" rel=\"noreferrer noopener\">PostgreSQL Self Join<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.oracletutorial.com\/oracle-basics\/oracle-self-join\/\" target=\"_blank\" rel=\"noreferrer noopener\">Oracle Self Join<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-self-join\/\" target=\"_blank\" rel=\"noreferrer noopener\">SQL Server Self Join<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.mysqltutorial.org\/mysql-basics\/mysql-self-join\/\" target=\"_blank\" rel=\"noreferrer noopener\">MySQL Self Join<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.sqlitetutorial.net\/sqlite-self-join\/\" target=\"_blank\" rel=\"noreferrer noopener\">SQLite Self Join<\/a><\/li>\n\n\n\n<li><a href=\"https:\/\/www.db2tutorial.com\/db2-basics\/db2-self-join\/\" target=\"_blank\" rel=\"noreferrer noopener\">Db2 Self Join<\/a><\/li>\n<\/ul>\n\n\n\n<p><\/p>\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 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