{"id":804,"date":"2019-06-09T18:55:49","date_gmt":"2019-06-10T02:55:49","guid":{"rendered":"https:\/\/db2tutorial.com\/?page_id=804"},"modified":"2020-04-11T06:58:09","modified_gmt":"2020-04-11T14:58:09","slug":"db2-unique-index","status":"publish","type":"page","link":"https:\/\/www.db2tutorial.com\/db2-index\/db2-unique-index\/","title":{"rendered":"Db2 UNIQUE Index"},"content":{"rendered":"\n<p><strong>Summary<\/strong>: in this tutorial, you will learn about Db2 unique indexes and how to use unique indexes to maintain data integrity by ensuring that no two rows in a table have duplicate key values.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Introduction to Db2 unique indexes<\/h2>\n\n\n\n<p>Unique indexes are indexes that allow you to maintain data integrity by ensuring that the index key columns do not contain any duplicate data.<\/p>\n\n\n\n<p>You can define a unique index on one or multiple columns of a table. If you include one column in the unique index, Db2 will enforce the uniqueness of values in this column. In case you define a unique index that includes multiple columns, Db2 will enforce the uniqueness of values in these columns.<\/p>\n\n\n\n<p>Any attempt to <a href=\"https:\/\/db2tutorial.com\/db2-basics\/db2-insert\/\">insert<\/a> or <a href=\"https:\/\/db2tutorial.com\/db2-basics\/db2-update\/\">update<\/a> data into the unique indexed columns that cause the duplicate will result in an error.<\/p>\n\n\n\n<p>When you create a unique index for the table that already has existing data, Db2 will check the data in the indexed key columns for the uniqueness. If the table contains duplicate key values, the index creation process will fail.<\/p>\n\n\n\n<p>Besides enforcing the uniqueness of data, unique indexes also help improve the speed of the queries that involve indexed key columns.<\/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=\"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> index_name\n<span class=\"hljs-keyword\">ON<\/span> table_name(c1,c2, ...);\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>In this syntax:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>First, specify the name of the unique index in the <code>CREATE UNIQUE INDEX<\/code> clause.<\/li><li>Second, specify the name of the table to which the unique index belongs and a list of columns that will be included in the index.<\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\">Db2 unique index examples<\/h2>\n\n\n\n<p>Let&#8217;s take some examples of using Db2 unique indexes.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">1) Creating a Db2 unique index on one column example<\/h3>\n\n\n\n<p>First, <a href=\"https:\/\/db2tutorial.com\/db2-basics\/db2-create-table\/\">create a table<\/a> that stores member 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> members(\n    member_id <span class=\"hljs-built_in\">INT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span> \n        <span class=\"hljs-keyword\">GENERATED<\/span> <span class=\"hljs-keyword\">ALWAYS<\/span> <span class=\"hljs-keyword\">AS<\/span> <span class=\"hljs-keyword\">IDENTITY<\/span> \n        PRIMARY <span class=\"hljs-keyword\">KEY<\/span>,\n    first_name <span class=\"hljs-built_in\">VARCHAR<\/span>(<span class=\"hljs-number\">100<\/span>) <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    last_name <span class=\"hljs-built_in\">VARCHAR<\/span>(<span class=\"hljs-number\">100<\/span>) <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    email <span class=\"hljs-built_in\">VARCHAR<\/span>(<span class=\"hljs-number\">255<\/span>) <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    joined_on <span class=\"hljs-built_in\">DATE<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span> <span class=\"hljs-keyword\">DEFAULT<\/span> <span class=\"hljs-keyword\">CURRENT<\/span> <span class=\"hljs-built_in\">DATE<\/span>\n);\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>Next, create a unique index that includes the <code>email<\/code> column to ensure the email is unique for every member.<\/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> ix_unq_email\n<span class=\"hljs-keyword\">ON<\/span> members(email);\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<figure class=\"wp-block-image\"><img loading=\"lazy\" decoding=\"async\" width=\"367\" height=\"218\" src=\"https:\/\/db2tutorial.com\/wp-content\/uploads\/2019\/06\/db2-unique-index-example.png\" alt=\"db2 unique index example\" class=\"wp-image-805\" srcset=\"https:\/\/www.db2tutorial.com\/wp-content\/uploads\/2019\/06\/db2-unique-index-example.png 367w, https:\/\/www.db2tutorial.com\/wp-content\/uploads\/2019\/06\/db2-unique-index-example-300x178.png 300w\" sizes=\"auto, (max-width: 367px) 100vw, 367px\" \/><\/figure>\n\n\n\n<p>Then, <a href=\"https:\/\/db2tutorial.com\/db2-basics\/db2-insert\/\">insert a new member<\/a> into the <code>members<\/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> members(first_name, last_name, email)\n<span class=\"hljs-keyword\">VALUES<\/span>(<span class=\"hljs-string\">'john'<\/span>,<span class=\"hljs-string\">'doe'<\/span>,<span class=\"hljs-string\">'john.doe@example.com'<\/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>After that, view the member data by using the following <code>SELECT<\/code> statement:<\/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\">SELECT<\/span> * <span class=\"hljs-keyword\">FROM<\/span> members;\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>Finally, attempt to insert a new member with the email that already exists:<\/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\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> members(first_name, last_name, email)\n<span class=\"hljs-keyword\">VALUES<\/span>(<span class=\"hljs-string\">'jane'<\/span>,<span class=\"hljs-string\">'doe'<\/span>,<span class=\"hljs-string\">'john.doe@example.com'<\/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>Db2 issued the following error:<\/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\">SQL Error &#91;23505]: One or more <span class=\"hljs-keyword\">values<\/span> <span class=\"hljs-keyword\">in<\/span> the <span class=\"hljs-keyword\">INSERT<\/span> <span class=\"hljs-keyword\">statement<\/span>, <span class=\"hljs-keyword\">UPDATE<\/span> <span class=\"hljs-keyword\">statement<\/span>, <span class=\"hljs-keyword\">or<\/span> <span class=\"hljs-keyword\">foreign<\/span> <span class=\"hljs-keyword\">key<\/span> <span class=\"hljs-keyword\">update<\/span> caused <span class=\"hljs-keyword\">by<\/span> a <span class=\"hljs-keyword\">DELETE<\/span> <span class=\"hljs-keyword\">statement<\/span> <span class=\"hljs-keyword\">are<\/span> <span class=\"hljs-keyword\">not<\/span> valid because the primary <span class=\"hljs-keyword\">key<\/span>, <span class=\"hljs-keyword\">unique<\/span> <span class=\"hljs-keyword\">constraint<\/span> <span class=\"hljs-keyword\">or<\/span> <span class=\"hljs-keyword\">unique<\/span> <span class=\"hljs-keyword\">index<\/span> <span class=\"hljs-keyword\">identified<\/span> <span class=\"hljs-keyword\">by<\/span> <span class=\"hljs-string\">\"2\"<\/span> constrains <span class=\"hljs-keyword\">table<\/span> <span class=\"hljs-string\">\"DB2ADMIN.MEMBERS\"<\/span> <span class=\"hljs-keyword\">from<\/span> <span class=\"hljs-keyword\">having<\/span> <span class=\"hljs-keyword\">duplicate<\/span> <span class=\"hljs-keyword\">values<\/span> <span class=\"hljs-keyword\">for<\/span> the <span class=\"hljs-keyword\">index<\/span> key.. \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<h3 class=\"wp-block-heading\">2) Creating a Db2 unique index on multiple columns example<\/h3>\n\n\n\n<p>First, create a <code>book_ratings<\/code> table that stores book rating data:<\/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\">CREATE<\/span> <span class=\"hljs-keyword\">TABLE<\/span> book_ratings(\n    rating_id <span class=\"hljs-built_in\">INT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span> \n        <span class=\"hljs-keyword\">GENERATED<\/span> <span class=\"hljs-keyword\">ALWAYS<\/span> <span class=\"hljs-keyword\">AS<\/span> <span class=\"hljs-keyword\">IDENTITY<\/span> \n        PRIMARY <span class=\"hljs-keyword\">KEY<\/span>, \n    rating <span class=\"hljs-built_in\">SMALLINT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    book_id <span class=\"hljs-built_in\">INT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    user_id <span class=\"hljs-built_in\">INT<\/span> <span class=\"hljs-keyword\">NOT<\/span> <span class=\"hljs-literal\">NULL<\/span>,\n    <span class=\"hljs-keyword\">CHECK<\/span>(rating &gt;= <span class=\"hljs-number\">1<\/span> <span class=\"hljs-keyword\">AND<\/span> rating &lt;= <span class=\"hljs-number\">5<\/span>),\n    <span class=\"hljs-keyword\">FOREIGN<\/span> <span class=\"hljs-keyword\">KEY<\/span>(book_ID) \n        <span class=\"hljs-keyword\">REFERENCES<\/span> books(book_id)\n);\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>Suppose, each user can rate a book one time only. To enforce this rule, you can create a unique index on the <code>book_id<\/code> and <code>user_id<\/code> columns.<\/p>\n\n\n\n<p>Second, create a unique index on the <code>book_id<\/code> and <code>user_id<\/code> columns:<\/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\">CREATE<\/span> <span class=\"hljs-keyword\">UNIQUE<\/span> <span class=\"hljs-keyword\">INDEX<\/span> rating_ip\n<span class=\"hljs-keyword\">ON<\/span> book_ratings(book_id, user_id);\n<\/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>Third, insert a rating into the <code>book_ratings<\/code> table:<\/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\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> \n    book_ratings(rating, book_id, user_id)\n<span class=\"hljs-keyword\">VALUES<\/span>\n    (<span class=\"hljs-number\">4<\/span>, <span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">20<\/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>In this example, the <code>user_id<\/code> 20 rated the book id 1 with 4 out of 5.<\/p>\n\n\n\n<p>Fourth, user id 20 attempts to vote the book id 1 one more time:<\/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\">INSERT<\/span> <span class=\"hljs-keyword\">INTO<\/span> book_ratings(rating, book_id, ip_address)\n<span class=\"hljs-keyword\">VALUES<\/span>(<span class=\"hljs-number\">5<\/span>, <span class=\"hljs-number\">1<\/span>, <span class=\"hljs-number\">20<\/span>);\n<\/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>Db2 issued an error.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Db2 unique index &amp; NULL treatments<\/h2>\n\n\n\n<p>Typically, <code>NULL<\/code> is not equal to itself. However, unique indexes treat <code>NULL<\/code> values equally. If a unique index has one column, this column can have only one NULL value.<\/p>\n\n\n\n<p>In case the unique index has multiple columns, these columns will have only one row that has <code>NULL<\/code> in every column.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Unique index vs. unique constraint<\/h2>\n\n\n\n<p>Both unique index and <a href=\"https:\/\/db2tutorial.com\/db2-basics\/db2-unique-constraint\/\">unique constraint<\/a> are used to enforce the uniqueness of data in one or multiple columns.<\/p>\n\n\n\n<p>When you create a unique constraint, Db2 automatically creates a unique index behind the scene and uses this index to enforce the uniqueness. Therefore, we can say that the unique constraints make the purposes of the unique indexes much more clear.<\/p>\n\n\n\n<p>In this tutorial, you have learned about the Db2 unique index and how to create a unique index for one or many columns of a table.<\/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=\"804\"\n\t\t\t\tdata-post-url=\"https:\/\/www.db2tutorial.com\/db2-index\/db2-unique-index\/\"\n\t\t\t\tdata-post-title=\"Db2 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 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