{"id":205754,"date":"2023-07-31T13:04:41","date_gmt":"2023-07-31T13:04:41","guid":{"rendered":"https:\/\/datascientest.com\/?p=205754"},"modified":"2024-06-03T10:25:35","modified_gmt":"2024-06-03T10:25:35","slug":"cle-etrangere-sql-tout-savoir","status":"publish","type":"post","link":"https:\/\/datascientest.com\/cle-etrangere-sql-tout-savoir","title":{"rendered":"Cl\u00e9 \u00e9trang\u00e8re ou foreign key en SQL : Qu&rsquo;est-ce que c&rsquo;est ? \u00c0 quoi \u00e7a sert ?"},"content":{"rendered":"\t\t<div data-elementor-type=\"wp-post\" data-elementor-id=\"205754\" class=\"elementor elementor-205754\" data-elementor-post-type=\"post\">\n\t\t\t\t\t\t\t<div class=\"elementor-element elementor-element-1d66fd6 e-flex e-con-boxed e-con\" data-id=\"1d66fd6\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-3858700 elementor-widget elementor-widget-heading\" data-id=\"3858700\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<style>\/*! elementor - v3.15.0 - 02-08-2023 *\/\n.elementor-heading-title{padding:0;margin:0;line-height:1}.elementor-widget-heading .elementor-heading-title[class*=elementor-size-]>a{color:inherit;font-size:inherit;line-height:inherit}.elementor-widget-heading .elementor-heading-title.elementor-size-small{font-size:15px}.elementor-widget-heading .elementor-heading-title.elementor-size-medium{font-size:19px}.elementor-widget-heading .elementor-heading-title.elementor-size-large{font-size:29px}.elementor-widget-heading .elementor-heading-title.elementor-size-xl{font-size:39px}.elementor-widget-heading .elementor-heading-title.elementor-size-xxl{font-size:59px}<\/style><h2 class=\"elementor-heading-title elementor-size-default\">Celui qui con\u00e7oit un syst\u00e8me de gestion de base de donn\u00e9es SQL a tout int\u00e9r\u00eat \u00e0 conna\u00eetre et \u00e0 mettre en \u0153uvre le syst\u00e8me des cl\u00e9s \u00e9trang\u00e8res. Sur le long terme, il sera largement gagnant.<\/h2>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-9e35768 elementor-widget elementor-widget-text-editor\" data-id=\"9e35768\" 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 - v3.15.0 - 02-08-2023 *\/\n.elementor-widget-text-editor.elementor-drop-cap-view-stacked .elementor-drop-cap{background-color:#69727d;color:#fff}.elementor-widget-text-editor.elementor-drop-cap-view-framed .elementor-drop-cap{color:#69727d;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><span style=\"font-weight: 400;\">Une cl\u00e9 \u00e9trang\u00e8re est une colonne (ou plusieurs colonnes) d\u2019une base de donn\u00e9es qui est reli\u00e9e \u00e0 la colonne cl\u00e9 primaire d\u2019une autre table. La cl\u00e9 primaire en question est un simple identifiant. L\u2019information utile &#8211; et susceptible d\u2019\u00e9voluer &#8211; figure dans la table externe.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-8496307 e-flex e-con-boxed e-con\" data-id=\"8496307\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-1b6d23c elementor-widget elementor-widget-heading\" data-id=\"1b6d23c\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Quelle utilit\u00e9 pour une cl\u00e9 \u00e9trang\u00e8re ?<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-169d81e elementor-widget elementor-widget-text-editor\" data-id=\"169d81e\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Imaginons, vous g\u00e9rez une table Articles dans laquelle figure une colonne appel\u00e9e Cat\u00e9gorie. Pour chaque article, cette cat\u00e9gorie doit \u00eatre <\/span><b>remplie individuellement<\/b><span style=\"font-weight: 400;\">.\u00a0<\/span><\/p><p><span style=\"font-weight: 400;\">\u00c0 titre d\u2019exemple, nous pourrions avoir parmi les cat\u00e9gories \u00ab Produit m\u00e9tallique \u00bb. Or, du jour au lendemain, la l\u00e9gislation change et d\u00e9sormais, cette cat\u00e9gorie devient \u00ab Produit \u00e0 base de m\u00e9taux \u00bb.\u00a0<\/span><\/p><p><span style=\"font-weight: 400;\">Que faire ? Va-t-il falloir modifier toutes les lignes dans lesquelles figure la cat\u00e9gorie \u00ab Produit m\u00e9tallique \u00bb ? H\u00e9las oui. Si la table Articles s\u2019\u00e9tend sur des dizaines de milliers d\u2019entr\u00e9es, la proc\u00e9dure va \u00eatre pour le moins fastidieuse.<\/span><\/p><p><span style=\"font-weight: 400;\">C\u2019est afin d\u2019\u00e9viter ce genre de situations que la notion de cl\u00e9 \u00e9trang\u00e8re a \u00e9t\u00e9 d\u00e9finie. Ainsi, dans le cas \u00e9voqu\u00e9 juste au-dessus, l\u2019id\u00e9e va consister \u00e0 cr\u00e9er une table s\u00e9par\u00e9e, appel\u00e9e Cat\u00e9gorie, dans laquelle nous aurons comme <\/span><b>cl\u00e9 primaire<\/b><span style=\"font-weight: 400;\">, un identifiant (un n\u00b0 pour chaque cat\u00e9gorie) associ\u00e9 \u00e0 la description correspondante.\u00a0<\/span><\/p><p><span style=\"font-weight: 400;\">\u00c0 partir de l\u00e0, dans la table Articles, il suffira d\u2019indiquer, pour chaque produit propos\u00e9 au catalogue, l\u2019<\/span><b>identifiant correspondant <\/b><span style=\"font-weight: 400;\">de la table Cat\u00e9gories. Il sera ensuite possible d\u2019imprimer la liste de chaque article tout en faisant appara\u00eetre (gr\u00e2ce \u00e0 une jointure) la cat\u00e9gorie correspondante.\u00a0<\/span><\/p><p><span style=\"font-weight: 400;\">Ainsi donc, comme nous pouvons le voir, la situation de la cat\u00e9gorie qui change de nom est ici r\u00e9solue.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-17b6a9c e-flex e-con-boxed e-con\" data-id=\"17b6a9c\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-2ad1723 elementor-widget elementor-widget-heading\" data-id=\"2ad1723\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Table parente et table fille<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-90be151 elementor-widget elementor-widget-text-editor\" data-id=\"90be151\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">On parle de \u00ab table parente \u00bb pour celle r\u00e9f\u00e9renc\u00e9e par une autre table. Ce serait ici le cas de la table Cat\u00e9gories.\u00a0<\/span><\/p><p><span style=\"font-weight: 400;\">Et l\u2019on parle de \u00ab table fille \u00bb pour qui fait r\u00e9f\u00e9rence \u00e0 une table externe \u2013 ce qui serait ici le cas de la table Articles.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-3bffc0d e-flex e-con-boxed e-con\" data-id=\"3bffc0d\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-d24eab0 elementor-widget elementor-widget-heading\" data-id=\"d24eab0\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Un exemple pratique<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-ddad87c elementor-widget elementor-widget-text-editor\" data-id=\"ddad87c\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Voici un autre exemple d\u2019usage d\u2019une cl\u00e9 \u00e9trang\u00e8re dans un cas de figure plus explicite.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-b468c8c elementor-widget elementor-widget-heading\" data-id=\"b468c8c\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h4 class=\"elementor-heading-title elementor-size-default\">Table CLIENTS<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-e03d0c4 elementor-widget elementor-widget-html\" data-id=\"e03d0c4\" data-element_type=\"widget\" data-widget_type=\"html.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<style type=\"text\/css\">\r\n.tg  {border-collapse:collapse;border-spacing:0;}\r\n.tg td{border-color:black;border-style:solid;border-width:1px;font-family:Arial, sans-serif;font-size:14px;\r\n  overflow:hidden;padding:10px 5px;word-break:normal;}\r\n.tg th{border-color:black;border-style:solid;border-width:1px;font-family:Arial, sans-serif;font-size:14px;\r\n  font-weight:normal;overflow:hidden;padding:10px 5px;word-break:normal;}\r\n.tg .tg-97aw{font-family:\"Trebuchet MS\", Helvetica, sans-serif !important;font-size:16px;text-align:center;vertical-align:top}\r\n.tg .tg-lyi1{font-family:\"Trebuchet MS\", Helvetica, sans-serif !important;font-size:16px;font-weight:bold;text-align:center;\r\n  vertical-align:top}\r\n<\/style>\r\n<table class=\"tg\" style=\"undefined;table-layout: fixed; width: 400px\">\r\n<colgroup>\r\n<col style=\"width: 100px\">\r\n<col style=\"width: 100px\">\r\n<col style=\"width: 100px\">\r\n<col style=\"width: 100px\">\r\n<\/colgroup>\r\n<thead>\r\n  <tr>\r\n    <th class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Id_Client<\/span><\/th>\r\n    <th class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Nom<\/span><br>\t\t<\/th>\r\n    <th class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Pr\u00e9nom<\/span><br>\t\t<\/th>\r\n    <th class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">\u00c2ge<\/span><br>\t\t<\/th>\r\n  <\/tr>\r\n<\/thead>\r\n<tbody>\r\n  <tr>\r\n    <td class=\"tg-lyi1\">1<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Martin<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Germaine<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">32<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">2<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Culprit<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Thomas<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">22<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">3<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Albanez<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Francis<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">40<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">4<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">De Jonqui\u00e8re<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Martine<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">33<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n<\/tbody>\r\n<\/table>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-0230dd6 elementor-widget elementor-widget-text-editor\" data-id=\"0230dd6\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">CLIENTS est ici la table \u00ab parente \u00bb. Si le client n\u00b04, Martine de Jonqui\u00e8re venait \u00e0 divorcer et reprenait demain son nom de jeune fille (Heras), il suffirait de modifier cette information dans la table \u00ab parente \u00bb.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-1fc051a elementor-align-center button-principale elementor-widget elementor-widget-button\" data-id=\"1fc051a\" data-element_type=\"widget\" data-widget_type=\"button.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<div class=\"elementor-button-wrapper\">\n\t\t\t<a class=\"elementor-button elementor-button-link elementor-size-sm elementor-animation-grow\" href=\"https:\/\/datascientest.com\/formation-sql\">\n\t\t\t\t\t\t<span class=\"elementor-button-content-wrapper\">\n\t\t\t\t\t\t<span class=\"elementor-button-text\">Tout comprendre des Cl\u00e9s \u00e9trang\u00e8res<\/span>\n\t\t<\/span>\n\t\t\t\t\t<\/a>\n\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-bfd5137 elementor-widget elementor-widget-heading\" data-id=\"bfd5137\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h4 class=\"elementor-heading-title elementor-size-default\">Table COMMANDES<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-9a93c3b elementor-widget elementor-widget-html\" data-id=\"9a93c3b\" data-element_type=\"widget\" data-widget_type=\"html.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<style type=\"text\/css\">\r\n.tg  {border-collapse:collapse;border-spacing:0;}\r\n.tg td{border-color:black;border-style:solid;border-width:1px;font-family:Arial, sans-serif;font-size:14px;\r\n  overflow:hidden;padding:10px 5px;word-break:normal;}\r\n.tg th{border-color:black;border-style:solid;border-width:1px;font-family:Arial, sans-serif;font-size:14px;\r\n  font-weight:normal;overflow:hidden;padding:10px 5px;word-break:normal;}\r\n.tg .tg-97aw{font-family:\"Trebuchet MS\", Helvetica, sans-serif !important;font-size:16px;text-align:center;vertical-align:top}\r\n.tg .tg-lyi1{font-family:\"Trebuchet MS\", Helvetica, sans-serif !important;font-size:16px;font-weight:bold;text-align:center;\r\n  vertical-align:top}\r\n<\/style>\r\n<table class=\"tg\" style=\"undefined;table-layout: fixed; width: 375px\">\r\n<colgroup>\r\n<col style=\"width: 125px\">\r\n<col style=\"width: 150px\">\r\n<col style=\"width: 100px\">\r\n<\/colgroup>\r\n<thead>\r\n  <tr>\r\n    <th class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Id_Commande<\/span><br>\t\t<\/th>\r\n    <th class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Date_Commande<\/span><br>\t\t<\/th>\r\n    <th class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">Id_Client<\/span><br>\t\t<\/th>\r\n  <\/tr>\r\n<\/thead>\r\n<tbody>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">1<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">11\/04\/2023<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">4<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">2<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">10\/05\/2023<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">3<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">3<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">12\/05\/2023<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">4<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">4<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">26\/05\/2023<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">4<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n  <tr>\r\n    <td class=\"tg-lyi1\"><span style=\"font-style:normal;text-decoration:none;color:#000;background-color:transparent\">5<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">16\/06\/2023<\/span><br>\t\t<\/td>\r\n    <td class=\"tg-97aw\"><span style=\"font-weight:400;font-style:normal;text-decoration:none;color:#000;background-color:transparent\">1<\/span><br>\t\t<\/td>\r\n  <\/tr>\r\n<\/tbody>\r\n<\/table>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-0919efc elementor-widget elementor-widget-text-editor\" data-id=\"0919efc\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Dans cet exemple, nous retrouvons la colonne \u00ab Id_Client \u00bb dans la table COMMANDES (qui est la table \u00ab fille \u00bb). Elle pointe vers la colonne \u00ab Id_Client \u00bb de la table CLIENTS. Ainsi, nous pourrions savoir ici au moyen d\u2019une jointure que Martine de Jonqui\u00e8res a pass\u00e9 trois fois commandes : une fois en avril et deux fois en juin.<\/span><\/p><p><span style=\"font-weight: 400;\">Dans la table CLIENTS, la colonne <strong>Id_Client<\/strong> est la cl\u00e9 primaire.<\/span><\/p><p><span style=\"font-weight: 400;\">Dans la table COMMANDES, la colonne <strong>Id_Client<\/strong> est la cl\u00e9 \u00e9trang\u00e8re.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-5047e54 e-flex e-con-boxed e-con\" data-id=\"5047e54\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-48dd001 elementor-widget elementor-widget-heading\" data-id=\"48dd001\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Avantages des cl\u00e9s \u00e9trang\u00e8res<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-f96281a elementor-widget elementor-widget-heading\" data-id=\"f96281a\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h4 class=\"elementor-heading-title elementor-size-default\">Int\u00e9grit\u00e9 r\u00e9f\u00e9rentielle<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-e21af77 elementor-widget elementor-widget-text-editor\" data-id=\"e21af77\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Le principal avantage li\u00e9 \u00e0 l\u2019usage des cl\u00e9s \u00e9trang\u00e8res est l\u2019int\u00e9grit\u00e9 r\u00e9f\u00e9rentielle : aucune<\/span> <span style=\"font-weight: 400;\"> ligne de la table fille ne sera compl\u00e8te et valide tant qu\u2019il n\u2019existe pas un enregistrement correspondant dans la table parent.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-de3c9b3 elementor-widget elementor-widget-heading\" data-id=\"de3c9b3\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h4 class=\"elementor-heading-title elementor-size-default\">Coh\u00e9rence des informations<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-eea432d elementor-widget elementor-widget-text-editor\" data-id=\"eea432d\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Toutes les tables qui font r\u00e9f\u00e9rence \u00e0 la cl\u00e9 \u00e9trang\u00e8re sont assur\u00e9es de pouvoir produire des donn\u00e9es coh\u00e9rentes et \u00e0 jour. Pour m\u00e9moire, si Martine de Jonqui\u00e8res change de nom, ce changement sera visible \u00e0 partir de toutes les tables faisant r\u00e9f\u00e9rence \u00e0 son identifiant.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-163f944 e-flex e-con-boxed e-con\" data-id=\"163f944\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-5656cf1 elementor-widget elementor-widget-heading\" data-id=\"5656cf1\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Inconv\u00e9nients des cl\u00e9s \u00e9trang\u00e8res<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-fba4ce9 elementor-widget elementor-widget-heading\" data-id=\"fba4ce9\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h4 class=\"elementor-heading-title elementor-size-default\">Vitesse de traitement amoindrie<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-bca7669 elementor-widget elementor-widget-text-editor\" data-id=\"bca7669\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">Bien \u00e9videmment, la cl\u00e9 \u00e9trang\u00e8re va impliquer un temps de traitement informatique plus long. Chaque fois que l\u2019on ins\u00e8re une nouvelle ligne dans une table telle que COMMANDES, il va \u00eatre n\u00e9cessaire de v\u00e9rifier si l\u2019identifiant client correspondant existe bel et bien dans la table CLIENTS.<\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-ad7ee4b elementor-widget elementor-widget-heading\" data-id=\"ad7ee4b\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h4 class=\"elementor-heading-title elementor-size-default\">Complexit\u00e9<\/h4>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-aef45da elementor-widget elementor-widget-text-editor\" data-id=\"aef45da\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<span style=\"font-weight: 400;\">Faire r\u00e9f\u00e9rence \u00e0 l\u2019id_client n\u00b04 peut appara\u00eetre plus complexe que de simplement indiquer le nom de Martine de Jonqui\u00e8res. Et donc, <i>a priori, <\/i>la lecture d\u2019une base de donn\u00e9es telles que celles des COMMANDES sera peu explicite au premier abord.<\/span>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-81df670 e-flex e-con-boxed e-con\" data-id=\"81df670\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-89ed8f5 elementor-widget elementor-widget-heading\" data-id=\"89ed8f5\" data-element_type=\"widget\" data-widget_type=\"heading.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<h3 class=\"elementor-heading-title elementor-size-default\">Un usage recommand\u00e9<\/h3>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-99fab34 elementor-widget elementor-widget-text-editor\" data-id=\"99fab34\" data-element_type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<p><span style=\"font-weight: 400;\">En d\u00e9pit des quelques inconv\u00e9nients que leur usage peut soulever, l\u2019usage des cl\u00e9s \u00e9trang\u00e8res est <\/span><b>fortement recommand\u00e9<\/b><span style=\"font-weight: 400;\"> pour celui qui a pour charge de concevoir des syst\u00e8mes de gestion bases de donn\u00e9es <a href=\"https:\/\/datascientest.com\/formation-sql\">SQL<\/a>. Sur le long terme, il sera largement gagnant. Pour faire simple, nous pourrions dire qu\u2019il importe d\u2019utiliser le syst\u00e8me des cl\u00e9s \u00e9trang\u00e8res chaque fois que nous avons affaire \u00e0 un type de donn\u00e9es susceptible d\u2019\u00e9voluer dans le temps. Les exemples abord\u00e9s ici illustrent clairement ce qui pourra \u00eatre gagn\u00e9. Et donc, <strong>il est crucial de d\u00e9finir les cl\u00e9s \u00e9trang\u00e8res d\u00e8s la conception m\u00eame d\u2019une base de donn\u00e9es SQL.<\/strong><\/span><\/p>\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-016d564 elementor-align-center button-principale elementor-widget elementor-widget-button\" data-id=\"016d564\" data-element_type=\"widget\" data-widget_type=\"button.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t<div class=\"elementor-button-wrapper\">\n\t\t\t<a class=\"elementor-button elementor-button-link elementor-size-sm elementor-animation-grow\" href=\"https:\/\/datascientest.com\/formation-sql\">\n\t\t\t\t\t\t<span class=\"elementor-button-content-wrapper\">\n\t\t\t\t\t\t<span class=\"elementor-button-text\">Apprendre \u00e0 g\u00e9rer des Bases SQL<\/span>\n\t\t<\/span>\n\t\t\t\t\t<\/a>\n\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-8156591 e-flex e-con-boxed e-con\" data-id=\"8156591\" data-element_type=\"container\" data-settings=\"{&quot;content_width&quot;:&quot;boxed&quot;}\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-315b8b7 elementor-widget elementor-widget-html\" data-id=\"315b8b7\" data-element_type=\"widget\" data-widget_type=\"html.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t<script type=\"application\/ld+json\">\r\n{\r\n  \"@context\": \"https:\/\/schema.org\",\r\n  \"@type\": \"FAQPage\",\r\n  \"mainEntity\": [{\r\n    \"@type\": \"Question\",\r\n    \"name\": \"Quelle utilit\u00e9 pour une cl\u00e9 \u00e9trang\u00e8re ?\",\r\n    \"acceptedAnswer\": {\r\n      \"@type\": \"Answer\",\r\n      \"text\": \"Imaginons, vous g\u00e9rez une table Articles dans laquelle figure une colonne appel\u00e9e Cat\u00e9gorie. Pour chaque article, cette cat\u00e9gorie doit \u00eatre remplie individuellement. \u00c0 titre d\u2019exemple, nous pourrions avoir parmi les cat\u00e9gories \u00ab Produit m\u00e9tallique \u00bb. Or, du jour au lendemain, la l\u00e9gislation change et d\u00e9sormais, cette cat\u00e9gorie devient \u00ab Produit \u00e0 base de m\u00e9taux \u00bb.\"\r\n    }\r\n  },{\r\n    \"@type\": \"Question\",\r\n    \"name\": \"Coh\u00e9rence des informations\",\r\n    \"acceptedAnswer\": {\r\n      \"@type\": \"Answer\",\r\n      \"text\": \"Toutes les tables qui font r\u00e9f\u00e9rence \u00e0 la cl\u00e9 \u00e9trang\u00e8re sont assur\u00e9es de pouvoir produire des donn\u00e9es coh\u00e9rentes et \u00e0 jour. Pour m\u00e9moire, si Martine de Jonqui\u00e8res change de nom, ce changement sera visible \u00e0 partir de toutes les tables faisant r\u00e9f\u00e9rence \u00e0 son identifiant.\"\r\n    }\r\n  },{\r\n    \"@type\": \"Question\",\r\n    \"name\": \"Vitesse de traitement amoindrie\",\r\n    \"acceptedAnswer\": {\r\n      \"@type\": \"Answer\",\r\n      \"text\": \"Bien \u00e9videmment, la cl\u00e9 \u00e9trang\u00e8re va impliquer un temps de traitement informatique plus long. Chaque fois que l\u2019on ins\u00e8re une nouvelle ligne dans une table telle que COMMANDES, il va \u00eatre n\u00e9cessaire de v\u00e9rifier si l\u2019identifiant client correspondant existe bel et bien dans la table CLIENTS.\"\r\n    }\r\n  }]\r\n}\r\n<\/script>\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"<p>Celui qui con\u00e7oit un syst\u00e8me de gestion de base de donn\u00e9es SQL a tout int\u00e9r\u00eat \u00e0 conna\u00eetre et \u00e0 mettre en \u0153uvre le syst\u00e8me des cl\u00e9s \u00e9trang\u00e8res. Sur le long terme, il sera largement gagnant. Une cl\u00e9 \u00e9trang\u00e8re est une colonne (ou plusieurs colonnes) d\u2019une base de donn\u00e9es qui est reli\u00e9e \u00e0 la colonne cl\u00e9 [&hellip;]<\/p>\n","protected":false},"author":78,"featured_media":210954,"comment_status":"open","ping_status":"open","sticky":false,"template":"elementor_theme","format":"standard","meta":{"content-type":"","inline_featured_image":false},"categories":[2497],"tags":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v21.7 (Yoast SEO v21.7) - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Cl\u00e9 \u00e9trang\u00e8re ou foreign key en SQL : Qu&#039;est-ce que c&#039;est ? \u00c0 quoi \u00e7a sert ?<\/title>\n<meta name=\"description\" content=\"Celui qui con\u00e7oit un syst\u00e8me de gestion de base de donn\u00e9es SQL a tout int\u00e9r\u00eat \u00e0 conna\u00eetre et \u00e0 mettre en \u0153uvre le syst\u00e8me des cl\u00e9s \u00e9trang\u00e8res. 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