{"id":3370,"date":"2024-04-27T20:45:27","date_gmt":"2024-04-27T13:45:27","guid":{"rendered":"https:\/\/www.sqlitetutorial.net\/?page_id=3370"},"modified":"2024-04-27T20:45:29","modified_gmt":"2024-04-27T13:45:29","slug":"sqlite-unique-index","status":"publish","type":"page","link":"https:\/\/www.sqlitetutorial.net\/sqlite-index\/sqlite-unique-index\/","title":{"rendered":"SQLite Unique Index"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you will learn how to create an SQLite unique index to ensure that values stored in a column or a set of columns are unique across the table.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Creating SQLite Unique Index<\/h2>\n\n\n\n<p>In SQLite, a unique index is an <a href=\"https:\/\/www.sqlitetutorial.net\/sqlite-index\/\">index<\/a> that ensures the values stored in a column or a set of columns are unique across the table.<\/p>\n\n\n\n<p>In other words, no two rows can have the same values in the indexed columns.<\/p>\n\n\n\n<p>To create a unique index, you use the <code>CREATE UNIQUE INDEX<\/code> statement as follows:<\/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\">CREATE<\/span> <span class=\"hljs-keyword\">UNIQUE<\/span> <span class=\"hljs-keyword\">INDEX<\/span> &#91;<span class=\"hljs-keyword\">IF<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-keyword\">EXISTS<\/span>] index_name\n<span class=\"hljs-keyword\">ON<\/span> table_name(column1, 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:<\/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.<\/li>\n\n\n\n<li>Second, use the <code>IF NOT EXISTS<\/code> option to create an index only if it does not exist.<\/li>\n\n\n\n<li>Third, provide the table name with which the unique index is associated.<\/li>\n\n\n\n<li>Finally, list out one or more index columns of the table inside the parentheses after the table name.<\/li>\n<\/ul>\n\n\n\n<p>Note that SQLite considers <code>NULLs<\/code> different. It means you can have multiple <code>NULLs<\/code> inside the columns of the unique index.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Creating a unique index for one column<\/h2>\n\n\n\n<p>First, <a href=\"https:\/\/www.sqlitetutorial.net\/sqlite-create-table\/\">create a table<\/a> called <code>contacts<\/code> to store the contact data:<\/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\">CREATE<\/span> <span class=\"hljs-keyword\">TABLE<\/span> contacts(\n    <span class=\"hljs-keyword\">id<\/span> <span class=\"hljs-built_in\">INTEGER<\/span> PRIMARY <span class=\"hljs-keyword\">KEY<\/span>,\n    <span class=\"hljs-keyword\">name<\/span> <span class=\"hljs-built_in\">TEXT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    email <span class=\"hljs-built_in\">TEXT<\/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\">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>Second, create a unique index on the <code>email<\/code> column of the <code>contacts<\/code> table:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-3\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">CREATE<\/span> <span class=\"hljs-keyword\">UNIQUE<\/span> <span class=\"hljs-keyword\">INDEX<\/span> contact_email\n<span class=\"hljs-keyword\">ON<\/span> contacts(email);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-3\"><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>Third, <a href=\"https:\/\/www.sqlitetutorial.net\/sqlite-insert\/\">insert a new row<\/a> into the <code>contacts<\/code> table:<\/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\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> contacts(<span class=\"hljs-keyword\">name<\/span>, email)\n<span class=\"hljs-keyword\">VALUES<\/span>(<span class=\"hljs-string\">'John Doe'<\/span>, <span class=\"hljs-string\">'john@test.com'<\/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>Fourth, insert another row into the <code>contacts<\/code> table with the email that already exists:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-5\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> contacts(<span class=\"hljs-keyword\">name<\/span>, email)\n<span class=\"hljs-keyword\">VALUES<\/span>(<span class=\"hljs-string\">'John Smith'<\/span>, <span class=\"hljs-string\">'john@test.com'<\/span>);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-5\"><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>SQLite rejected the change and issued the following error:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-6\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\">Runtime error: UNIQUE constraint failed: contacts.email (19)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-6\"><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>The error indicates that the unique index works as expected.<\/p>\n\n\n\n<p>Fifth, insert two more <code>contacts<\/code> without emails:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-7\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> contacts(<span class=\"hljs-keyword\">name<\/span>, email)\n<span class=\"hljs-keyword\">VALUES<\/span> (<span class=\"hljs-string\">'Alice'<\/span>, <span class=\"hljs-literal\">NULL<\/span>),\n       (<span class=\"hljs-string\">'Bob'<\/span>, <span class=\"hljs-literal\">NULL<\/span>);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-7\"><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>Finally, query data from the <code>contacts<\/code> table:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-8\" 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> * <span class=\"hljs-keyword\">FROM<\/span> contacts;<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-8\"><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>Output:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-9\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\">id  name      email\n<span class=\"hljs-comment\">--  --------  -------------<\/span>\n1   John Doe  john@test.com\n2   Alice     null\n3   Bob       null<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-9\"><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>The output indicates that SQLite allows multiple <code>NULLs<\/code> in the column of the unique index.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Creating a unique index for multiple columns<\/h2>\n\n\n\n<p>First, create a table called <code>calendars<\/code> to store month and year data:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-10\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">CREATE<\/span> <span class=\"hljs-keyword\">TABLE<\/span> calendars (\n    <span class=\"hljs-keyword\">id<\/span> <span class=\"hljs-built_in\">INTEGER<\/span> PRIMARY <span class=\"hljs-keyword\">KEY<\/span>,\n    <span class=\"hljs-keyword\">year<\/span> <span class=\"hljs-built_in\">INT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    <span class=\"hljs-keyword\">month<\/span> <span class=\"hljs-built_in\">INT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span> <span class=\"hljs-keyword\">CHECK<\/span> (<span class=\"hljs-keyword\">month<\/span> &gt;= <span class=\"hljs-number\">1<\/span> <span class=\"hljs-keyword\">and<\/span> <span class=\"hljs-keyword\">month<\/span> &lt;=<span class=\"hljs-number\">12<\/span>),\n    month_name <span class=\"hljs-built_in\">TEXT<\/span> <span class=\"hljs-keyword\">GENERATED<\/span> <span class=\"hljs-keyword\">ALWAYS<\/span> <span class=\"hljs-keyword\">AS<\/span> (\n        <span class=\"hljs-keyword\">CASE<\/span> <span class=\"hljs-keyword\">month<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">1<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'January'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">2<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'February'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">3<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'March'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">4<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'April'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">5<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'May'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">6<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'June'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">7<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'July'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">8<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'August'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">9<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'September'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">10<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'October'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">11<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'November'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">12<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'December'<\/span>\n        <span class=\"hljs-keyword\">END<\/span>\n    ) <span class=\"hljs-keyword\">STORED<\/span>,\n    month_abbr <span class=\"hljs-built_in\">TEXT<\/span> <span class=\"hljs-keyword\">GENERATED<\/span> <span class=\"hljs-keyword\">ALWAYS<\/span> <span class=\"hljs-keyword\">AS<\/span> (\n        <span class=\"hljs-keyword\">CASE<\/span> <span class=\"hljs-keyword\">month<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">1<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Jan'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">2<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Feb'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">3<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Mar'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">4<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Apr'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">5<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'May'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">6<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Jun'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">7<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Jul'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">8<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Aug'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">9<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Sep'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">10<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Oct'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">11<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Nov'<\/span>\n            <span class=\"hljs-keyword\">WHEN<\/span> <span class=\"hljs-number\">12<\/span> <span class=\"hljs-keyword\">THEN<\/span> <span class=\"hljs-string\">'Dec'<\/span>\n        <span class=\"hljs-keyword\">END<\/span>\n    ) <span class=\"hljs-keyword\">STORED<\/span>\n);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-10\"><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>Second, create a unique index on the <code>year<\/code> and <code>month<\/code> columns:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-11\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">CREATE<\/span> <span class=\"hljs-keyword\">UNIQUE<\/span> <span class=\"hljs-keyword\">INDEX<\/span> calendar_year_month\n<span class=\"hljs-keyword\">ON<\/span> calendars(<span class=\"hljs-keyword\">year<\/span>, <span class=\"hljs-keyword\">month<\/span>);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-11\"><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>Third, insert rows into the <code>calendars<\/code> table:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-12\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> calendars(<span class=\"hljs-keyword\">year<\/span>, <span class=\"hljs-keyword\">month<\/span>)\n<span class=\"hljs-keyword\">VALUES<\/span>\n   (<span class=\"hljs-number\">2024<\/span>,<span class=\"hljs-number\">1<\/span>),\n   (<span class=\"hljs-number\">2024<\/span>,<span class=\"hljs-number\">2<\/span>),\n   (<span class=\"hljs-number\">2024<\/span>,<span class=\"hljs-number\">3<\/span>);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-12\"><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>Fourth, retrieve data from the calendars table:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-13\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\">id  year  month  month_name  month_abbr\n<span class=\"hljs-comment\">--  ----  -----  ----------  ----------<\/span>\n1   2024  1      January     Jan\n2   2024  2      February    Feb\n3   2024  3      March       Mar<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-13\"><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>The output indicates that SQLite uses the values in the year and month columns to evaluate the uniqueness.<\/p>\n\n\n\n<p>Fifth, attempt to insert a year and month that already exists:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-14\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\"><span class=\"hljs-keyword\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> calendars(<span class=\"hljs-keyword\">year<\/span>, <span class=\"hljs-keyword\">month<\/span>)\n<span class=\"hljs-keyword\">VALUES<\/span> (<span class=\"hljs-number\">2024<\/span>,<span class=\"hljs-number\">3<\/span>);<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-14\"><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>SQLite rejected the changes and issued the following error:<\/p>\n\n\n<pre class=\"wp-block-code\" aria-describedby=\"shcb-language-15\" data-shcb-language-name=\"SQL (Structured Query Language)\" data-shcb-language-slug=\"sql\"><span><code class=\"hljs language-sql\">Runtime error: UNIQUE constraint failed: calendars.year, calendars.month (19)<\/code><\/span><small class=\"shcb-language\" id=\"shcb-language-15\"><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<h2 class=\"wp-block-heading\">Summary<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Use <code>CREATE UNIQUE INDEX<\/code> statement to create a unique index on one or more columns of a table.<\/li>\n\n\n\n<li>SQLite treats <code>NULLs<\/code> differently. It means you can have multiple <code>NULLs<\/code> in columns of a unique index.<\/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=\"3370\"\n\t\t\t\tdata-post-url=\"https:\/\/www.sqlitetutorial.net\/sqlite-index\/sqlite-unique-index\/\"\n\t\t\t\tdata-post-title=\"SQLite Unique Index\"\n\t\t\t\tdata-response=\"1\"\n\t\t\t\tclass=\"wth-btn-rounded wth-yes-btn\"\n\t\t\t>\n\t\t\t\t<svg\n\t\t\t\t\txmlns=\"http:\/\/www.w3.org\/2000\/svg\"\n\t\t\t\t\tviewBox=\"0 0 24 24\"\n\t\t\t\t\tfill=\"none\"\n\t\t\t\t\tstroke=\"currentColor\"\n\t\t\t\t\tstroke-width=\"2\"\n\t\t\t\t\tstroke-linecap=\"round\"\n\t\t\t\t\tstroke-linejoin=\"round\"\n\t\t\t\t\tclass=\"feather feather-thumbs-up block w-full h-full\"\n\t\t\t\t>\n\t\t\t\t\t<path\n\t\t\t\t\t\td=\"M14 9V5a3 3 0 0 0-3-3l-4 9v11h11.28a2 2 0 0 0 2-1.7l1.38-9a2 2 0 0 0-2-2.3zM7 22H4a2 2 0 0 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