{"id":27414,"date":"2024-03-16T08:26:24","date_gmt":"2024-03-16T08:26:24","guid":{"rendered":"https:\/\/www.silicloud.com\/blog\/how-to-set-unique-constraints-when-creating-a-table-in-sql\/"},"modified":"2024-03-22T10:58:49","modified_gmt":"2024-03-22T10:58:49","slug":"how-to-set-unique-constraints-when-creating-a-table-in-sql","status":"publish","type":"post","link":"https:\/\/www.silicloud.com\/blog\/how-to-set-unique-constraints-when-creating-a-table-in-sql\/","title":{"rendered":"How to set unique constraints when creating a table in SQL?"},"content":{"rendered":"<p>In SQL, you can use the UNIQUE keyword to set a unique constraint. This constraint ensures that the values in a column or a few columns in a table are unique, meaning that these column values cannot be duplicated in the entire table. When creating a table, you can use the UNIQUE keyword at the end of the column definition to set a unique constraint. For example, when creating a table named &#8220;users&#8221; with a column named &#8220;username,&#8221; you can use the following syntax to set a unique constraint:<\/p>\n<pre class=\"post-pre\">CREATE&nbsp;TABLE&nbsp;users&nbsp;(<p><\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;id&nbsp;INT&nbsp;PRIMARY&nbsp;KEY,<\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;username&nbsp;VARCHAR(255)&nbsp;UNIQUE,<\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;...<\/p><p>);<\/p><\/pre>\n<p>In the above example, the &#8220;username&#8221; column is set as a unique constraint, which means that the values in the &#8220;username&#8221; column of any two rows in the table cannot be the same. When trying to insert or update data, if the unique constraint is violated, the database will throw an error. If you want to set a unique constraint for multiple columns, you can use multi-column unique constraints. For example, when creating a table named &#8220;users&#8221; with columns &#8220;username&#8221; and &#8220;email&#8221;, you can use the following syntax to set a multi-column unique constraint:<\/p>\n<pre class=\"post-pre\">CREATE&nbsp;TABLE&nbsp;users&nbsp;(<p><\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;id&nbsp;INT&nbsp;PRIMARY&nbsp;KEY,<\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;username&nbsp;VARCHAR(255),<\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;email&nbsp;VARCHAR(255),<\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;UNIQUE&nbsp;(username,&nbsp;email),<\/p><p><\/p><p>&nbsp;&nbsp;&nbsp;&nbsp;...<\/p><p>);<\/p><\/pre>\n<p>In the example above, the columns &#8220;username&#8221; and &#8220;email&#8221; have been set as a unique constraint, which means that the combination of values in these two columns for any two rows in the table cannot be the same. If an attempt is made to insert or update data that violates this unique constraint, the database will throw an error.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>In SQL, you can use the UNIQUE keyword to set a unique constraint. This constraint ensures that the values in a column or a few columns in a table are unique, meaning that these column values cannot be duplicated in the entire table. When creating a table, you can use the UNIQUE keyword at the [&hellip;]<\/p>\n","protected":false},"author":13,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_import_markdown_pro_load_document_selector":0,"_import_markdown_pro_submit_text_textarea":"","footnotes":""},"categories":[1],"tags":[],"class_list":["post-27414","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v21.5 (Yoast SEO v21.5) - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>How to set unique constraints when creating a table in SQL? - Blog - Silicon Cloud<\/title>\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.silicloud.com\/blog\/how-to-set-unique-constraints-when-creating-a-table-in-sql\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How to set unique constraints when creating a table in SQL?\" \/>\n<meta property=\"og:description\" content=\"In SQL, you can use the UNIQUE keyword to set a unique constraint. 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