{"id":2222,"date":"2025-03-04T10:59:49","date_gmt":"2025-03-04T03:59:49","guid":{"rendered":"https:\/\/www.pgtutorial.com\/?page_id=2222"},"modified":"2025-03-07T11:11:55","modified_gmt":"2025-03-07T04:11:55","slug":"postgresql-unique-index","status":"publish","type":"page","link":"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-unique-index\/","title":{"rendered":"PostgreSQL Unique Index"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you&#8217;ll learn how to use the PostgreSQL unique index to ensure the values in one or more table columns are unique.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='postgresql-unique-index-overview'>PostgreSQL Unique Index Overview <a href=\"#postgresql-unique-index-overview\" class=\"anchor\" id=\"postgresql-unique-index-overview\" title=\"Anchor for PostgreSQL Unique Index Overview\">#<\/a><\/h2>\n\n\n\n<p>A unique <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-index\/\">index<\/a> ensures values in one or more columns are unique across all rows in a table, maintaining the integrity of your data.<\/p>\n\n\n\n<p>You&#8217;ll encounter an error if you attempt to insert a value already in a column with a unique index.<\/p>\n\n\n\n<p>To create a unique index, you use the <code>CREATE UNIQUE INDEX<\/code> statement:<\/p>\n\n\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\">CREATE<\/span> <span class=\"hljs-keyword\">UNIQUE<\/span> <span class=\"hljs-keyword\">INDEX<\/span> &#91;index_name]\n<span class=\"hljs-keyword\">ON<\/span> <span class=\"hljs-built_in\">table_name<\/span> (column1&#91;, column2, ...])\n&#91; NULLS &#91; <span class=\"hljs-keyword\">NOT<\/span> ] <span class=\"hljs-keyword\">DISTINCT<\/span> ];<\/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>\n\n\n<p>In this syntax:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>First, specify the index name after the <code>CREATE UNIQUE INDEX<\/code> keywords. If you omit it, PostgreSQL will automatically generate a name.<\/li>\n\n\n\n<li>Second, provide the table name on which you want to create the index.<\/li>\n\n\n\n<li>Third, list one or more columns included in the index.<\/li>\n\n\n\n<li>Third, the <code>NULL NOT DISTINCT<\/code>&nbsp;treats NULLs equally, while&nbsp;the <code>NULLS DISTINCT<\/code>&nbsp;considers NULLs as distinct values. The default is <code>NULLS DISTINCT<\/code>, meaning the index column may contain multiple NULLs.<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id='postgresql-unique-index-example'>PostgreSQL Unique Index Example <a href=\"#postgresql-unique-index-example\" class=\"anchor\" id=\"postgresql-unique-index-example\" title=\"Anchor for PostgreSQL Unique Index Example\">#<\/a><\/h2>\n\n\n\n<p>First, create a table <code>warehouse_workers<\/code> to store the warehouse worker data:<\/p>\n\n\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> <span class=\"hljs-keyword\">IF<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-keyword\">EXISTS<\/span> warehouse_workers (\n  id <span class=\"hljs-type\">INT<\/span> <span class=\"hljs-keyword\">GENERATED<\/span> <span class=\"hljs-keyword\">ALWAYS<\/span> <span class=\"hljs-keyword\">AS<\/span> <span class=\"hljs-keyword\">IDENTITY<\/span> <span class=\"hljs-keyword\">PRIMARY KEY<\/span>,\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>,\n  email <span class=\"hljs-type\">VARCHAR<\/span>(<span class=\"hljs-number\">255<\/span>),\n  phone <span class=\"hljs-type\">VARCHAR<\/span>(<span class=\"hljs-number\">25<\/span>),\n  <span class=\"hljs-keyword\">extension<\/span> <span class=\"hljs-type\">VARCHAR<\/span>(<span class=\"hljs-number\">25<\/span>)\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>\n\n\n<p>Second, create a unique index for the <code>email<\/code> column:<\/p>\n\n\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\">CREATE<\/span> <span class=\"hljs-keyword\">UNIQUE<\/span> <span class=\"hljs-keyword\">INDEX<\/span> <span class=\"hljs-keyword\">ON<\/span> warehouse_workers (email);<\/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>\n\n\n<p>This unique index ensures the uniqueness of values in the <code>email<\/code> column of the table.<\/p>\n\n\n\n<p>Third, <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-insert\/\">insert a new row<\/a> into the <code>warehouse_workers<\/code> table:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-4\" 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>\n  warehouse_workers (<span class=\"hljs-type\">name<\/span>, email, phone, <span class=\"hljs-keyword\">extension<\/span>)\n<span class=\"hljs-keyword\">VALUES<\/span>\n  (\n    <span class=\"hljs-string\">'Emma Smith'<\/span>,\n    <span class=\"hljs-string\">'emma.s@pgtutorial.com'<\/span>,\n    <span class=\"hljs-string\">'(408)-111-2222'<\/span>,\n    <span class=\"hljs-string\">'101'<\/span>\n  );<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-4\"><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>\n\n\n<p>Finally, attempt to insert a new row with the same email address:<\/p>\n\n\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\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span>\n  warehouse_workers (<span class=\"hljs-type\">name<\/span>, email, phone, <span class=\"hljs-keyword\">extension<\/span>)\n<span class=\"hljs-keyword\">VALUES<\/span>\n  (\n    <span class=\"hljs-string\">'Emma Scott'<\/span>,\n    <span class=\"hljs-string\">'emma.s@pgtutorial.com'<\/span>,\n    <span class=\"hljs-string\">'(408)-111-2222'<\/span>,\n    <span class=\"hljs-string\">'102'<\/span>\n  );<\/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>\n\n\n<p>PostgreSQL issued an error:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-6\" data-shcb-language-name=\"PostgreSQL SQL dialect and PL\/pgSQL\" data-shcb-language-slug=\"pgsql\"><span><code class=\"hljs language-pgsql\">ERROR: duplicate key <span class=\"hljs-keyword\">value<\/span> violates <span class=\"hljs-keyword\">unique<\/span> <span class=\"hljs-keyword\">constraint<\/span> \"warehouse_workers_email_idx\"\nDETAIL: Key (email)=(emma.s@pgtutorial.com) already <span class=\"hljs-keyword\">exists<\/span>.<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-6\"><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>\n\n\n<p>The unique index <code>warehouse_workers_email_idx<\/code> does not allow duplicate emails in the <code>email<\/code> column.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='creating-a-unique-index-on-multiple-columns'>Creating a unique index on multiple columns <a href=\"#creating-a-unique-index-on-multiple-columns\" class=\"anchor\" id=\"creating-a-unique-index-on-multiple-columns\" title=\"Anchor for Creating a unique index on multiple columns\">#<\/a><\/h2>\n\n\n\n<p>First, create a unique index that includes the <code>phone<\/code> and <code>extension<\/code> columns:<\/p>\n\n\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\">CREATE<\/span> <span class=\"hljs-keyword\">UNIQUE<\/span> <span class=\"hljs-keyword\">INDEX<\/span> <span class=\"hljs-keyword\">ON<\/span> warehouse_workers (phone, <span class=\"hljs-keyword\">extension<\/span>);<\/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>\n\n\n<p>Second, <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-insert\/\">insert a new row<\/a> into the <code>warehouse_workers<\/code> table:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-8\" 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>\n  warehouse_workers (<span class=\"hljs-type\">name<\/span>, email, phone, <span class=\"hljs-keyword\">extension<\/span>)\n<span class=\"hljs-keyword\">VALUES<\/span>\n  (\n    <span class=\"hljs-string\">'Mary Scott'<\/span>,\n    <span class=\"hljs-string\">'mary.s@pgtutorial.com'<\/span>,\n    <span class=\"hljs-string\">'(408)-111-2222'<\/span>,\n    <span class=\"hljs-string\">'102'<\/span>\n  );<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-8\"><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>\n\n\n<p>Even though the phone number of Mary is the same as Emma&#8217;s, the statement successfully inserted a row into the table.<\/p>\n\n\n\n<p>The reason is that the new unique index enforces the unique values in the <code>phone<\/code> and <code>extension<\/code> columns.<\/p>\n\n\n\n<p>Third, attempt to insert a new row with the same phone and extension:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-9\" 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>\n  warehouse_workers (<span class=\"hljs-type\">name<\/span>, email, phone, <span class=\"hljs-keyword\">extension<\/span>)\n<span class=\"hljs-keyword\">VALUES<\/span>\n  (\n    <span class=\"hljs-string\">'Ava Garcia\u00a0'<\/span>,\n    <span class=\"hljs-string\">'ava.g@pgtutorial.com'<\/span>,\n    <span class=\"hljs-string\">'(408)-111-2222'<\/span>,\n    <span class=\"hljs-string\">'102'<\/span>\n  );<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-9\"><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>\n\n\n<p>PostgreSQL issued the following error:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-10\" data-shcb-language-name=\"PostgreSQL SQL dialect and PL\/pgSQL\" data-shcb-language-slug=\"pgsql\"><span><code class=\"hljs language-pgsql\">ERROR: duplicate key <span class=\"hljs-keyword\">value<\/span> violates <span class=\"hljs-keyword\">unique<\/span> <span class=\"hljs-keyword\">constraint<\/span> \"warehouse_workers_phone_extension_idx\"\nDETAIL: Key (phone, <span class=\"hljs-keyword\">extension<\/span>)=((<span class=\"hljs-number\">408<\/span>)<span class=\"hljs-number\">-111<\/span><span class=\"hljs-number\">-2222<\/span>, <span class=\"hljs-number\">102<\/span>) already <span class=\"hljs-keyword\">exists<\/span>.<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-10\"><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>\n\n\n<p>The reason is that the pair of values <code>(408)-111-2222, 102)<\/code> already exists in the <code>warehouse_workers<\/code> table.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='unique-index-vs-primary-key'>Unique index vs. primary key <a href=\"#unique-index-vs-primary-key\" class=\"anchor\" id=\"unique-index-vs-primary-key\" title=\"Anchor for Unique index vs. primary key\">#<\/a><\/h2>\n\n\n\n<p>In PostgreSQL, a primary key is a special kind of unique index that:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Does not allow NULLs.<\/li>\n\n\n\n<li>Can be applied to one or more columns per table.<\/li>\n\n\n\n<li>Implies uniqueness inherently<\/li>\n<\/ul>\n\n\n\n<p>In contrast, a unique index allows NULLs unless you specify <code>NULL NOT DISTINCT<\/code> option.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id='unique-index-vs-unique-constraint'>Unique index vs. unique constraint <a href=\"#unique-index-vs-unique-constraint\" class=\"anchor\" id=\"unique-index-vs-unique-constraint\" title=\"Anchor for Unique index vs. unique constraint\">#<\/a><\/h2>\n\n\n\n<p>Both unique index and <a href=\"https:\/\/www.pgtutorial.com\/postgresql-tutorial\/postgresql-unique-constraint\/\">unique constraint<\/a> ensure the uniqueness of values in one or more columns within a table.<\/p>\n\n\n\n<p>However, a unique constraint is a logical schema rule, while a unique index is a physical structure that PostgreSQL uses to enforce the uniqueness.<\/p>\n\n\n\n<p>PostgreSQL automatically generates a unique index when you create a unique constraint.<\/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>Use a unique index to enforce the uniqueness values in one or more columns.<\/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=unique-index\"\n  height=\"700\"\n  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