{"id":1629,"date":"2023-05-02T16:46:48","date_gmt":"2023-05-02T16:46:48","guid":{"rendered":"http:\/\/localhost\/codeera\/?p=1629"},"modified":"2023-05-02T17:06:06","modified_gmt":"2023-05-02T17:06:06","slug":"understanding-primary-keys-foreign-keys-and-unique-constraints-in-sql","status":"publish","type":"post","link":"http:\/\/localhost\/codeera\/2023\/05\/02\/understanding-primary-keys-foreign-keys-and-unique-constraints-in-sql\/","title":{"rendered":"Understanding Primary Keys, Foreign Keys, and Unique Constraints in SQL"},"content":{"rendered":"\t\t<div data-elementor-type=\"wp-post\" data-elementor-id=\"1629\" class=\"elementor elementor-1629\">\n\t\t\t\t\t\t\t\t\t<section class=\"elementor-section elementor-top-section elementor-element elementor-element-fab2917 elementor-section-boxed elementor-section-height-default elementor-section-height-default\" data-id=\"fab2917\" data-element_type=\"section\">\n\t\t\t\t\t\t<div class=\"elementor-container elementor-column-gap-default\">\n\t\t\t\t\t<div class=\"elementor-column elementor-col-100 elementor-top-column elementor-element elementor-element-c10d15e\" data-id=\"c10d15e\" data-element_type=\"column\">\n\t\t\t<div class=\"elementor-widget-wrap elementor-element-populated\">\n\t\t\t\t\t\t\t\t<div class=\"elementor-element elementor-element-9b8f4d4 elementor-widget elementor-widget-text-editor\" data-id=\"9b8f4d4\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<style>\/*! elementor - v3.11.1 - 15-02-2023 *\/\n.elementor-widget-text-editor.elementor-drop-cap-view-stacked .elementor-drop-cap{background-color:#818a91;color:#fff}.elementor-widget-text-editor.elementor-drop-cap-view-framed .elementor-drop-cap{color:#818a91;border:3px solid;background-color:transparent}.elementor-widget-text-editor:not(.elementor-drop-cap-view-default) .elementor-drop-cap{margin-top:8px}.elementor-widget-text-editor:not(.elementor-drop-cap-view-default) .elementor-drop-cap-letter{width:1em;height:1em}.elementor-widget-text-editor .elementor-drop-cap{float:left;text-align:center;line-height:1;font-size:50px}.elementor-widget-text-editor .elementor-drop-cap-letter{display:inline-block}<\/style>\t\t\t\t<p><!-- wp:list {\"ordered\":true} --><\/p><ol><li style=\"list-style-type: none;\"><ol><!-- wp:list-item --><\/ol><\/li><\/ol><p><b>\u00a0<\/b>In SQL, primary keys, foreign keys, and unique constraints are used to ensure data integrity and consistency.\u00a0<br \/><br \/><\/p><h5><strong>1. Primary Key:<\/strong><\/h5><p>A primary key is a column or a set of columns in a table that uniquely identifies each row in the table. It is used to enforce data integrity and ensure that there are no duplicate rows in the table. A primary key must be unique, not null, and it should not change over time.<\/p><p>The syntax for defining a primary key is as follows:<br \/><br \/><\/p><pre class=\"wp-block-preformatted\"><code><strong>CREATE TABLE table_name (<\/strong><br \/><strong>column1 datatype PRIMARY KEY,<\/strong><br \/><strong>column2 datatype,<\/strong><br \/><strong>...<\/strong><br \/><strong>);\n<\/strong><\/code><\/pre><h5>2. Foreign Key:<\/h5><p>A foreign key is a column or a set of columns in a table that refers to the primary key of another table. It is used to establish a relationship between two tables and enforce referential integrity. A foreign key must have a matching primary key value in the referenced table or have a null value.<\/p><p>The syntax for defining a foreign key is as follows:<br \/><br \/><\/p><p><!-- \/wp:paragraph --><!-- wp:preformatted --><\/p><pre class=\"wp-block-preformatted\"><code><b>    CREATE TABLE table_name (<br \/>column1 datatype,<br \/>column2 datatype,<br \/>FOREIGN KEY (column1)<br \/>REFERENCES referenced_table_name (referenced_column1, referenced_column2, ...);<br \/>);<\/b><\/code><\/pre><h5>3. Unique Constraint:<\/h5><p>A unique constraint is a rule that ensures that the values in a column or a set of columns are unique. It is used to enforce data integrity and prevent duplicate values in a table. A unique constraint can be applied to one or more columns in a table. <br \/><br \/>The syntax for defining a unique constraint is as follows:<\/p><div class=\"bg-black rounded-md mb-4\"><div class=\"flex items-center relative text-gray-200 bg-gray-800 px-4 py-2 text-xs font-sans justify-between rounded-t-md\">\u00a0<\/div><\/div><pre class=\"wp-block-preformatted\"><code><b> CREATE TABLE table_name (<br \/>column1 datatype,<br \/>column2 datatype,<br \/>CONSTRAINT constraint_name UNIQUE (column1, column2, ...);<br \/>);   <\/b><\/code><\/pre><p>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 In summary, primary keys are used to uniquely identify rows in a table, foreign keys are used to establish relationships between tables, and unique constraints are used to ensure the uniqueness of values in one or more columns in a table. Understanding these concepts is essential for creating well-designed and structured databases.<\/p><p>\u00a0<\/p><p><!-- \/wp:paragraph --><!-- wp:paragraph --><\/p><p><!-- \/wp:paragraph --><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"default","ast-global-header-display":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","footnotes":""},"categories":[1],"tags":[],"_links":{"self":[{"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/posts\/1629"}],"collection":[{"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/comments?post=1629"}],"version-history":[{"count":13,"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/posts\/1629\/revisions"}],"predecessor-version":[{"id":1646,"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/posts\/1629\/revisions\/1646"}],"wp:attachment":[{"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/media?parent=1629"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/categories?post=1629"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/localhost\/codeera\/wp-json\/wp\/v2\/tags?post=1629"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}