{"id":1826,"date":"2019-04-23T09:44:36","date_gmt":"2019-04-23T02:44:36","guid":{"rendered":"http:\/\/www.sqlservertutorial.net\/?page_id=1826"},"modified":"2020-07-13T07:34:44","modified_gmt":"2020-07-13T00:34:44","slug":"sql-server-find-duplicates","status":"publish","type":"page","link":"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-find-duplicates\/","title":{"rendered":"Find Duplicates From a Table in SQL Server"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you will learn how to use the <code>GROUP BY<\/code> clause or <code>ROW_NUMBER()<\/code> function to find duplicate values in a table.<\/p>\n\n\n\n<p>Technically, you use the <code><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-unique-constraint\/\">UNIQUE<\/a><\/code> constraints to enforce the uniqueness of rows in one or more columns of a table. However, sometimes you may find duplicate values in a table due to the poor database design, application bugs, or uncleaned data from external sources. Your job is to identify these duplicate values in effective ways.<\/p>\n\n\n\n<p>To find the duplicate values in a table, you follow these steps:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>First, define criteria for duplicates: values in a single column or multiple columns.<\/li><li>Second, write a query to search for duplicates.<\/li><\/ul>\n\n\n\n<p>If you want to also delete the duplicate rows, you can go to the <a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/delete-duplicates-sql-server\/\">deleting duplicates from a table tutorial<\/a>.<\/p>\n\n\n\n<p>Let&#8217;s set up a sample table for the demonstration.<\/p>\n\n\n\n<h2 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><\/h2>\n\n\n\n<p>First, <a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-create-table\/\">create a new table<\/a> named <code>t1<\/code> that contains three columns <code>id<\/code>, <code>a<\/code>, and <code>b<\/code>.<\/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\">DROP<\/span> <span class=\"hljs-keyword\">TABLE<\/span> <span class=\"hljs-keyword\">IF<\/span> <span class=\"hljs-keyword\">EXISTS<\/span> t1;\n<span class=\"hljs-keyword\">CREATE<\/span> <span class=\"hljs-keyword\">TABLE<\/span> t1 (\n    <span class=\"hljs-keyword\">id<\/span> <span class=\"hljs-built_in\">INT<\/span> <span class=\"hljs-keyword\">IDENTITY<\/span>(<span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">1<\/span>), \n    a  <span class=\"hljs-built_in\">INT<\/span>, \n    b  <span class=\"hljs-built_in\">INT<\/span>, \n    PRIMARY <span class=\"hljs-keyword\">KEY<\/span>(<span class=\"hljs-keyword\">id<\/span>)\n);\n<\/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>Then, <a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-insert\/\">insert<\/a> some rows into the <code>t1<\/code> table:<\/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\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span>\n    t1(a,b)\n<span class=\"hljs-keyword\">VALUES<\/span>\n    (<span class=\"hljs-number\">1<\/span>,<span class=\"hljs-number\">1<\/span>),\n    (<span class=\"hljs-number\">1<\/span>,<span class=\"hljs-number\">2<\/span>),\n    (<span class=\"hljs-number\">1<\/span>,<span class=\"hljs-number\">3<\/span>),\n    (<span class=\"hljs-number\">2<\/span>,<span class=\"hljs-number\">1<\/span>),\n    (<span class=\"hljs-number\">1<\/span>,<span class=\"hljs-number\">2<\/span>),\n    (<span class=\"hljs-number\">1<\/span>,<span class=\"hljs-number\">3<\/span>),\n    (<span class=\"hljs-number\">2<\/span>,<span class=\"hljs-number\">1<\/span>),\n    (<span class=\"hljs-number\">2<\/span>,<span class=\"hljs-number\">2<\/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>The <code>t1<\/code> table contains the following duplicate rows:<\/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\">(1,2)\n(2,1)\n(1,3)\n<\/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>Your goal is to write a query to find the above duplicate rows.<\/p>\n\n\n\n\n\n\n<h2 class=\"wp-block-heading\" id='using-group-by-clause-to-find-duplicates-in-a-table'>Using GROUP BY clause to find duplicates in a table <a href=\"#using-group-by-clause-to-find-duplicates-in-a-table\" class=\"anchor\" id=\"using-group-by-clause-to-find-duplicates-in-a-table\" title=\"Anchor for Using &lt;code&gt;GROUP BY&lt;\/code&gt; clause to find duplicates in a table\">#<\/a><\/h2>\n\n\n\n<p>This statement uses the <code><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-group-by\/\">GROUP BY<\/a><\/code> clause to find the duplicate rows in both <code>a<\/code> and <code>b<\/code> columns of the <code>t1<\/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\">SELECT<\/span> \n    a, \n    b, \n    <span class=\"hljs-keyword\">COUNT<\/span>(*) occurrences\n<span class=\"hljs-keyword\">FROM<\/span> t1\n<span class=\"hljs-keyword\">GROUP<\/span> <span class=\"hljs-keyword\">BY<\/span>\n    a, \n    b\n<span class=\"hljs-keyword\">HAVING<\/span> \n    <span class=\"hljs-keyword\">COUNT<\/span>(*) &gt; <span class=\"hljs-number\">1<\/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\">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>Here is the result:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"126\" height=\"73\" src=\"https:\/\/www.sqlservertutorial.net\/wp-content\/uploads\/SQL-Server-Find-Duplicates-using-GROUP-BY-clause.png\" alt=\"SQL Server Find Duplicates using GROUP BY clause\" class=\"wp-image-1837\"\/><\/figure>\n\n\n\n<p>How it works:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>First, the <a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-group-by\/\"><code>GROUP BY<\/code><\/a> clause groups the rows into groups by values in both <code>a<\/code> and <code>b<\/code> columns.<\/li><li>Second, the <code><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-aggregate-functions\/sql-server-count\/\">COUNT()<\/a><\/code> function returns the number of occurrences of each group (a,b).<\/li><li>Third, the <code><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-having\/\">HAVING<\/a><\/code> clause keeps only duplicate groups, which are groups that have more than one occurrence.<\/li><\/ul>\n\n\n\n<p>To return the entire row for each duplicate row, you <a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-inner-join\/\">join<\/a> the result of the above query with the t1 table using a <a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-cte\/\">common table expression<\/a> (<a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-cte\/\">CTE<\/a>):<\/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\">WITH<\/span> cte <span class=\"hljs-keyword\">AS<\/span> (\n    <span class=\"hljs-keyword\">SELECT<\/span>\n        a, \n        b, \n        <span class=\"hljs-keyword\">COUNT<\/span>(*) occurrences\n    <span class=\"hljs-keyword\">FROM<\/span> t1\n    <span class=\"hljs-keyword\">GROUP<\/span> <span class=\"hljs-keyword\">BY<\/span> \n        a, \n        b\n    <span class=\"hljs-keyword\">HAVING<\/span> \n        <span class=\"hljs-keyword\">COUNT<\/span>(*) &gt; <span class=\"hljs-number\">1<\/span>\n)\n<span class=\"hljs-keyword\">SELECT<\/span> \n    t1.id, \n    t1.a, \n    t1.b\n<span class=\"hljs-keyword\">FROM<\/span> t1\n    <span class=\"hljs-keyword\">INNER<\/span> <span class=\"hljs-keyword\">JOIN<\/span> cte <span class=\"hljs-keyword\">ON<\/span> \n        cte.a = t1.a <span class=\"hljs-keyword\">AND<\/span> \n        cte.b = t1.b\n<span class=\"hljs-keyword\">ORDER<\/span> <span class=\"hljs-keyword\">BY<\/span> \n    t1.a, \n    t1.b;\n<\/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>Here is the output:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"73\" height=\"133\" src=\"https:\/\/www.sqlservertutorial.net\/wp-content\/uploads\/SQL-Server-Find-Duplicates-using-GROUP-BY-clause-Result.png\" alt=\"\" class=\"wp-image-1839\"\/><\/figure>\n\n\n\n<p>Generally, the query for finding the duplicate values in one column using the <code><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-group-by\/\">GROUP BY<\/a><\/code> clause is as follows:<\/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\"><span class=\"hljs-keyword\">SELECT<\/span> \n    <span class=\"hljs-keyword\">col<\/span>, \n    <span class=\"hljs-keyword\">COUNT<\/span>(<span class=\"hljs-keyword\">col<\/span>)\n<span class=\"hljs-keyword\">FROM<\/span> \n    table_name\n<span class=\"hljs-keyword\">GROUP<\/span> <span class=\"hljs-keyword\">BY<\/span> \n    <span class=\"hljs-keyword\">col<\/span>\n<span class=\"hljs-keyword\">HAVING<\/span> \n    <span class=\"hljs-keyword\">COUNT<\/span>(<span class=\"hljs-keyword\">col<\/span>) &gt; <span class=\"hljs-number\">1<\/span>;\n<\/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 query for finding the duplicate values in multiple columns using the <code>GROUP BY<\/code> clause :<\/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\">SELECT<\/span> \n    col1,col2,...\n    <span class=\"hljs-keyword\">COUNT<\/span>(*)\n<span class=\"hljs-keyword\">FROM<\/span> \n    table_name\n<span class=\"hljs-keyword\">GROUP<\/span> <span class=\"hljs-keyword\">BY<\/span> \n    col1,col2,...\n<span class=\"hljs-keyword\">HAVING<\/span> \n    <span class=\"hljs-keyword\">COUNT<\/span>(*) &gt; <span class=\"hljs-number\">1<\/span>;\n<\/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<h2 class=\"wp-block-heading\" id='using-row_number-function-to-find-duplicates-in-a-table'>Using ROW_NUMBER() function to find duplicates in a table <a href=\"#using-row_number-function-to-find-duplicates-in-a-table\" class=\"anchor\" id=\"using-row_number-function-to-find-duplicates-in-a-table\" title=\"Anchor for Using &lt;code&gt;ROW_NUMBER()&lt;\/code&gt; function to find duplicates in a table\">#<\/a><\/h2>\n\n\n\n<p>The following statement uses the <code><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-window-functions\/sql-server-row_number-function\/\">ROW_NUMBER()<\/a><\/code> function to find duplicate rows based on both <code>a<\/code> and <code>b<\/code> columns:<\/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\">WITH<\/span> cte <span class=\"hljs-keyword\">AS<\/span> (\n    <span class=\"hljs-keyword\">SELECT<\/span> \n        a, \n        b, \n        ROW_NUMBER() <span class=\"hljs-keyword\">OVER<\/span> (\n            <span class=\"hljs-keyword\">PARTITION<\/span> <span class=\"hljs-keyword\">BY<\/span> a,b\n            <span class=\"hljs-keyword\">ORDER<\/span> <span class=\"hljs-keyword\">BY<\/span> a,b) <span class=\"hljs-keyword\">rownum<\/span>\n    <span class=\"hljs-keyword\">FROM<\/span> \n        t1\n) \n<span class=\"hljs-keyword\">SELECT<\/span> \n  * \n<span class=\"hljs-keyword\">FROM<\/span> \n    cte \n<span class=\"hljs-keyword\">WHERE<\/span> \n    <span class=\"hljs-keyword\">rownum<\/span> &gt; <span class=\"hljs-number\">1<\/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\">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>Here is the result:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"108\" height=\"70\" src=\"https:\/\/www.sqlservertutorial.net\/wp-content\/uploads\/SQL-Server-Find-Duplicates-using-ROW_NUMBER.png\" alt=\"SQL Server Find Duplicates using ROW_NUMBER\" class=\"wp-image-1840\"\/><\/figure>\n\n\n\n<p>How it works:<\/p>\n\n\n\n<p>First, the <code><a href=\"https:\/\/www.sqlservertutorial.net\/sql-server-window-functions\/sql-server-row_number-function\/\">ROW_NUMBER()<\/a><\/code> distributes rows of the t1 table into partitions by values in the <code>a<\/code> and <code>b<\/code> columns. The duplicate rows will have repeated values in the <code>a<\/code> and <code>b<\/code> columns, but different row numbers as shown in the following picture:<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"108\" height=\"172\" src=\"https:\/\/www.sqlservertutorial.net\/wp-content\/uploads\/SQL-Server-Find-Duplicates-using-ROW_NUMBER-Result.png\" alt=\"\" class=\"wp-image-1842\"\/><\/figure>\n\n\n\n<p>Second, the outer query removes the first row in each group.<\/p>\n\n\n\n<p>Generally, This statement uses the <code>ROW_NUMBER()<\/code> function to find the duplicate values in one column of a table:<\/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\"><span class=\"hljs-keyword\">WITH<\/span> cte <span class=\"hljs-keyword\">AS<\/span> (\n    <span class=\"hljs-keyword\">SELECT<\/span> \n        <span class=\"hljs-keyword\">col<\/span>,\n        ROW_NUMBER() <span class=\"hljs-keyword\">OVER<\/span> (\n            <span class=\"hljs-keyword\">PARTITION<\/span> <span class=\"hljs-keyword\">BY<\/span> <span class=\"hljs-keyword\">col<\/span>\n            <span class=\"hljs-keyword\">ORDER<\/span> <span class=\"hljs-keyword\">BY<\/span> <span class=\"hljs-keyword\">col<\/span>) row_num\n    <span class=\"hljs-keyword\">FROM<\/span> \n        t1\n) \n<span class=\"hljs-keyword\">SELECT<\/span> * <span class=\"hljs-keyword\">FROM<\/span> cte \n<span class=\"hljs-keyword\">WHERE<\/span> row_num &gt; <span class=\"hljs-number\">1<\/span>;<\/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>In this tutorial, you have learned how to use the <code>GROUP BY<\/code> clause or <code>ROW_NUMBER()<\/code> function to find duplicate values in SQL Server.<\/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 class=\"wth-thumbs\">\n\t\t\t<button\n\t\t\t\tdata-post=\"1826\"\n\t\t\t\tdata-post-url=\"https:\/\/www.sqlservertutorial.net\/sql-server-basics\/sql-server-find-duplicates\/\"\n\t\t\t\tdata-post-title=\"Find Duplicates From a Table in SQL Server\"\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 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