{"id":326,"date":"2024-11-25T17:34:57","date_gmt":"2024-11-25T10:34:57","guid":{"rendered":"https:\/\/www.pgtutorial.com\/?page_id=326"},"modified":"2025-02-10T14:58:18","modified_gmt":"2025-02-10T07:58:18","slug":"postgresql-self-join","status":"publish","type":"page","link":"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-self-join\/","title":{"rendered":"PostgreSQL Self-join"},"content":{"rendered":"\r\n<p><strong>Summary<\/strong>: in this tutorial, you&#8217;ll learn how to use PostgreSQL self-join to compare rows within the same table.<\/p>\r\n\r\n\r\n\r\n<h2 class=\"wp-block-heading\" id='introduction-to-postgresql-self-join'>Introduction to PostgreSQL Self-join <a href=\"#introduction-to-postgresql-self-join\" class=\"anchor\" id=\"introduction-to-postgresql-self-join\" title=\"Anchor for Introduction to PostgreSQL Self-join\">#<\/a><\/h2>\r\n\r\n\r\n\r\n<p>PostgreSQL self-join is a powerful technique that allows you to join a table to itself using an <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-inner-join\/\">inner join<\/a>, <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-left-join\/\">left join<\/a>, or <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-right-join\/\">right join<\/a>.<\/p>\r\n\r\n\r\n\r\n<p>Since a table is joined to itself, this kind of join technique is called <em>self-join<\/em>. The self-join technique can be helpful when you want to compare rows within the same table.<\/p>\r\n\r\n\r\n\r\n<p>PostgreSQL does not allow you to use the same table multiple times within the same SQL statement. If you do so, PostgreSQL will issue a syntax error.<\/p>\r\n\r\n\r\n\r\n<p>To use the same table multiple times within a statement, you must assign the table different aliases. In this case, PostgreSQL will treat these aliases as separate tables.<\/p>\r\n\r\n\r\n\r\n<p>Here&#8217;s the syntax of a self-join:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-1\" data-shcb-language-name=\"PostgreSQL SQL dialect and PL\/pgSQL\" data-shcb-language-slug=\"pgsql\"><span><code class=\"hljs language-pgsql\"><span class=\"hljs-keyword\">SELECT<\/span>\r\n  column1,\r\n  column2,\r\n  column3\r\n<span class=\"hljs-keyword\">FROM<\/span>\r\n  <span class=\"hljs-built_in\">table_name<\/span> t1\r\n  <span class=\"hljs-keyword\">INNER<\/span> <span class=\"hljs-keyword\">JOIN<\/span> <span class=\"hljs-built_in\">table_name<\/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\">PostgreSQL SQL dialect and PL\/pgSQL<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">pgsql<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\r\n\r\n\r\n<p>In this syntax:<\/p>\r\n\r\n\r\n\r\n<ul class=\"wp-block-list\">\r\n<li>First, specify a table name in the <code>FROM<\/code> clause with the alias <code>t1<\/code>.<\/li>\r\n\r\n\r\n\r\n<li>Second, provide the same table name in the <code>INNER JOIN<\/code> clause with the alias <code>t2<\/code>.<\/li>\r\n\r\n\r\n\r\n<li>Third, use a condition to match rows within the table.<\/li>\r\n<\/ul>\r\n\r\n\r\n\r\n<p>Besides the <code>INNER JOIN<\/code> clause, you can use <code>LEFT JOIN<\/code> or <code>RIGHT JOIN<\/code> clause in the <code>SELECT<\/code> statement.<\/p>\r\n\r\n\r\n\r\n<h2 class=\"wp-block-heading\" id='postgresql-self-join-examples'>PostgreSQL Self-Join examples <a href=\"#postgresql-self-join-examples\" class=\"anchor\" id=\"postgresql-self-join-examples\" title=\"Anchor for PostgreSQL Self-Join examples\">#<\/a><\/h2>\r\n\r\n\r\n\r\n<p>Let&#8217;s take some examples of using PostgreSQL self-join.<\/p>\r\n\r\n\r\n\r\n<h3 class=\"wp-block-heading\" id='setting-up-a-sample-table'>Setting up a sample table <a href=\"#setting-up-a-sample-table\" class=\"anchor\" id=\"setting-up-a-sample-table\" title=\"Anchor for Setting up a sample table\">#<\/a><\/h3>\r\n\r\n\r\n\r\n<p>First, <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-create-table\/\">create a new table<\/a> called <code>categories<\/code>:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-2\" data-shcb-language-name=\"PostgreSQL SQL dialect and PL\/pgSQL\" data-shcb-language-slug=\"pgsql\"><span><code class=\"hljs language-pgsql\"><span class=\"hljs-keyword\">CREATE<\/span> <span class=\"hljs-keyword\">TABLE<\/span> categories (\r\n  category_id <span class=\"hljs-type\">INT<\/span> <span class=\"hljs-keyword\">GENERATED<\/span> <span class=\"hljs-keyword\">BY<\/span> <span class=\"hljs-keyword\">DEFAULT<\/span> <span class=\"hljs-keyword\">AS<\/span> <span class=\"hljs-keyword\">IDENTITY<\/span> <span class=\"hljs-keyword\">PRIMARY KEY<\/span>,\r\n  <span class=\"hljs-type\">name<\/span> <span class=\"hljs-type\">VARCHAR<\/span>(<span class=\"hljs-number\">255<\/span>) <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-keyword\">NULL<\/span>,\r\n  parent_id <span class=\"hljs-type\">INT<\/span>,\r\n  <span class=\"hljs-keyword\">FOREIGN KEY<\/span> (parent_id) <span class=\"hljs-keyword\">REFERENCES<\/span> categories (category_id) <span class=\"hljs-keyword\">ON<\/span> <span class=\"hljs-keyword\">DELETE<\/span> <span class=\"hljs-keyword\">CASCADE<\/span>\r\n);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-2\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">PostgreSQL SQL dialect and PL\/pgSQL<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">pgsql<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\r\n\r\n\r\n<p>In the <code>categories<\/code> table, the <code>parent_id<\/code> is a <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-foreign-key\/\">foreign key column<\/a> that references the <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-primary-key\/\">primary key column<\/a> <code>category_id<\/code>.<\/p>\r\n\r\n\r\n\r\n<p>The top category will have <code>NULL<\/code> in the <code>parent_id<\/code> column.<\/p>\r\n\r\n\r\n\r\n<p>A subcategory will have a value in the <code>parent_id<\/code> column that is the same as a value in the <code>category_id<\/code> column of an existing row.<\/p>\r\n\r\n\r\n\r\n<p>Second, <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-insert\/\">insert data<\/a> into the <code>categories<\/code> table:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-3\" data-shcb-language-name=\"PostgreSQL SQL dialect and PL\/pgSQL\" data-shcb-language-slug=\"pgsql\"><span><code class=\"hljs language-pgsql\"><span class=\"hljs-keyword\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span>\r\n  categories (category_id, <span class=\"hljs-type\">name<\/span>, parent_id)\r\n<span class=\"hljs-keyword\">VALUES<\/span>\r\n  (<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-string\">'Electronics'<\/span>, <span class=\"hljs-keyword\">NULL<\/span>),\r\n  (<span class=\"hljs-number\">2<\/span>, <span class=\"hljs-string\">'Mobile Devices'<\/span>, <span class=\"hljs-number\">1<\/span>),\r\n  (<span class=\"hljs-number\">3<\/span>, <span class=\"hljs-string\">'Smartphones'<\/span>, <span class=\"hljs-number\">2<\/span>),\r\n  (<span class=\"hljs-number\">4<\/span>, <span class=\"hljs-string\">'Tablets'<\/span>, <span class=\"hljs-number\">2<\/span>),\r\n  (<span class=\"hljs-number\">5<\/span>, <span class=\"hljs-string\">'Accessories'<\/span>, <span class=\"hljs-number\">2<\/span>),\r\n  (<span class=\"hljs-number\">6<\/span>, <span class=\"hljs-string\">'Wearables'<\/span>, <span class=\"hljs-number\">2<\/span>),\r\n  (<span class=\"hljs-number\">7<\/span>, <span class=\"hljs-string\">'Home Entertainment'<\/span>, <span class=\"hljs-number\">1<\/span>),\r\n  (<span class=\"hljs-number\">8<\/span>, <span class=\"hljs-string\">'Televisions'<\/span>, <span class=\"hljs-number\">7<\/span>),\r\n  (<span class=\"hljs-number\">9<\/span>, <span class=\"hljs-string\">'Audio Systems'<\/span>, <span class=\"hljs-number\">7<\/span>),\r\n  (<span class=\"hljs-number\">10<\/span>, <span class=\"hljs-string\">'Computers'<\/span>, <span class=\"hljs-number\">1<\/span>),\r\n  (<span class=\"hljs-number\">11<\/span>, <span class=\"hljs-string\">'Laptops'<\/span>, <span class=\"hljs-number\">10<\/span>),\r\n  (<span class=\"hljs-number\">12<\/span>, <span class=\"hljs-string\">'Desktops'<\/span>, <span class=\"hljs-number\">10<\/span>)\r\n<span class=\"hljs-keyword\">RETURNING<\/span> *;<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-3\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">PostgreSQL SQL dialect and PL\/pgSQL<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">pgsql<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\r\n\r\n\r\n<p>Output:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-4\" data-shcb-language-name=\"plaintext\" data-shcb-language-slug=\"plaintext\"><span><code class=\"hljs language-plaintext\"> category_id |        name        | parent_id\r\n-------------+--------------------+-----------\r\n           1 | Electronics        |      NULL\r\n           2 | Mobile Devices     |         1\r\n           3 | Smartphones        |         2\r\n           4 | Tablets            |         2\r\n           5 | Accessories        |         2\r\n           6 | Wearables          |         2\r\n           7 | Home Entertainment |         1\r\n           8 | Televisions        |         7\r\n           9 | Audio Systems      |         7\r\n          10 | Computers          |         1\r\n          11 | Laptops            |        10\r\n          12 | Desktops           |        10<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-4\"><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>\r\n\r\n\r\n<h3 class=\"wp-block-heading\" id='postgresql-self-join-using-an-inner-join'>PostgreSQL self-join using an inner join <a href=\"#postgresql-self-join-using-an-inner-join\" class=\"anchor\" id=\"postgresql-self-join-using-an-inner-join\" title=\"Anchor for PostgreSQL self-join using an inner join\">#<\/a><\/h3>\r\n\r\n\r\n\r\n<p>The following statement uses an <code>INNER JOIN<\/code> to select categories and their parents:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-5\" data-shcb-language-name=\"PostgreSQL SQL dialect and PL\/pgSQL\" data-shcb-language-slug=\"pgsql\"><span><code class=\"hljs language-pgsql\"><span class=\"hljs-keyword\">SELECT<\/span>\r\n  c.name category,\r\n  p.name parent\r\n<span class=\"hljs-keyword\">FROM<\/span>\r\n  categories c\r\n  <span class=\"hljs-keyword\">INNER<\/span> <span class=\"hljs-keyword\">JOIN<\/span> categories p <span class=\"hljs-keyword\">ON<\/span> c.parent_id = p.category_id;<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-5\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">PostgreSQL SQL dialect and PL\/pgSQL<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">pgsql<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\r\n\r\n\r\n<p><a href=\"https:\/\/www.pgtutorial.com\/playground\/?db=selfjoin&amp;q=U0VMRUNUIGMubmFtZSBjYXRlZ29yeSwgcC5uYW1lIHBhcmVudCBGUk9NIGNhdGVnb3JpZXMgYyBJTk5FUiBKT0lOIGNhdGVnb3JpZXMgcCBPTiBjLnBhcmVudF9pZCA9IHAuY2F0ZWdvcnlfaWQ7\" target=\"_blank\" rel=\"noreferrer noopener\">Try it<\/a><\/p>\r\n\r\n\r\n\r\n<p>Output:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-6\" data-shcb-language-name=\"plaintext\" data-shcb-language-slug=\"plaintext\"><span><code class=\"hljs language-plaintext\">      category      |       parent\r\n--------------------+--------------------\r\n Mobile Devices     | Electronics\r\n Smartphones        | Mobile Devices\r\n Tablets            | Mobile Devices\r\n Accessories        | Mobile Devices\r\n Wearables          | Mobile Devices\r\n Home Entertainment | Electronics\r\n Televisions        | Home Entertainment\r\n Audio Systems      | Home Entertainment\r\n Computers          | Electronics\r\n Laptops            | Computers\r\n Desktops           | Computers<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-6\"><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>\r\n\r\n\r\n<p>The output does not show the top-level category, which has the <code>parent_id<\/code> <code>NULL<\/code>. To include the top-level category, you need to use a <code>LEFT JOIN<\/code> to perform a self-join.<\/p>\r\n\r\n\r\n\r\n<h3 class=\"wp-block-heading\" id='postgresql-self-join-using-a-left-join'>PostgreSQL self-join using a left join <a href=\"#postgresql-self-join-using-a-left-join\" class=\"anchor\" id=\"postgresql-self-join-using-a-left-join\" title=\"Anchor for PostgreSQL self-join using a left join\">#<\/a><\/h3>\r\n\r\n\r\n\r\n<p>The following statement uses a <code>LEFT JOIN<\/code> to select categories and their parents, including the top category:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-7\" data-shcb-language-name=\"PostgreSQL SQL dialect and PL\/pgSQL\" data-shcb-language-slug=\"pgsql\"><span><code class=\"hljs language-pgsql\"><span class=\"hljs-keyword\">SELECT<\/span>\r\n  c.name category,\r\n  p.name parent\r\n<span class=\"hljs-keyword\">FROM<\/span>\r\n  categories c\r\n  <span class=\"hljs-keyword\">LEFT JOIN<\/span> categories p <span class=\"hljs-keyword\">ON<\/span> c.parent_id = p.category_id;<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-7\"><span class=\"shcb-language__label\">Code language:<\/span> <span class=\"shcb-language__name\">PostgreSQL SQL dialect and PL\/pgSQL<\/span> <span class=\"shcb-language__paren\">(<\/span><span class=\"shcb-language__slug\">pgsql<\/span><span class=\"shcb-language__paren\">)<\/span><\/small><\/pre>\r\n\r\n\r\n<p><a href=\"https:\/\/www.pgtutorial.com\/playground\/?db=selfjoin&amp;q=U0VMRUNUIGMubmFtZSBjYXRlZ29yeSwgcC5uYW1lIHBhcmVudCBGUk9NIGNhdGVnb3JpZXMgYyBMRUZUIEpPSU4gY2F0ZWdvcmllcyBwIE9OIGMucGFyZW50X2lkID0gcC5jYXRlZ29yeV9pZDs\" target=\"_blank\" rel=\"noreferrer noopener\">Try it<\/a><\/p>\r\n\r\n\r\n\r\n<p>Output:<\/p>\r\n\r\n\r\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-8\" data-shcb-language-name=\"plaintext\" data-shcb-language-slug=\"plaintext\"><span><code class=\"hljs language-plaintext\">      category      |       parent\r\n--------------------+--------------------\r\n Electronics        | NULL\r\n Mobile Devices     | Electronics\r\n Smartphones        | Mobile Devices\r\n Tablets            | Mobile Devices\r\n Accessories        | Mobile Devices\r\n Wearables          | Mobile Devices\r\n Home Entertainment | Electronics\r\n Televisions        | Home Entertainment\r\n Audio Systems      | Home Entertainment\r\n Computers          | Electronics\r\n Laptops            | Computers\r\n Desktops           | Computers<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-8\"><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>\r\n\r\n\r\n<h2 class=\"wp-block-heading\" id='summary'>Summary <a href=\"#summary\" class=\"anchor\" id=\"summary\" title=\"Anchor for Summary\">#<\/a><\/h2>\r\n\r\n\r\n\r\n<ul class=\"wp-block-list\">\r\n<li>A self-join is a join where a table is joined to itself.<\/li>\r\n\r\n\r\n\r\n<li>Use a self-join when you want to compare rows within the same table.<\/li>\r\n<\/ul>\r\n\r\n\r\n\r\n<h2 class=\"wp-block-heading\" id='quiz'>Quiz <a href=\"#quiz\" class=\"anchor\" id=\"quiz\" title=\"Anchor for Quiz\">#<\/a><\/h2>\r\n\r\n\r\n\r\n<p><iframe loading=\"lazy\" class=\"iframe\" src=\"\/quiz\/?quiz=self-join\" name=\"quiz\" width=\"600\" height=\"700\"><\/iframe><\/p>\r\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=\"326\"\n\t\t\t\tdata-post-url=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-self-join\/\"\n\t\t\t\tdata-post-title=\"PostgreSQL Self-join\"\n\t\t\t\tdata-response=\"1\"\n\t\t\t\tclass=\"wth-btn-rounded 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table.<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":13,"menu_order":26,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-326","page","type-page","status-publish","hentry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>PostgreSQL Self-join<\/title>\n<meta name=\"description\" content=\"In this tutorial, you&#039;ll learn how to use PostgreSQL self-join to join a table to itself using an inner join, left join, and right join.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-self-join\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"PostgreSQL Self-join\" 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