{"id":477274,"date":"2023-08-09T09:10:23","date_gmt":"2023-08-09T09:10:23","guid":{"rendered":""},"modified":"2023-09-05T11:14:24","modified_gmt":"2023-09-05T11:14:24","slug":"foreign-key","status":"publish","type":"wiki","link":"https:\/\/oneproxy.pro\/vn\/wiki\/foreign-key\/","title":{"rendered":"Kh\u00f3a ngo\u1ea1i"},"content":{"rendered":"<p>Kh\u00f3a ngo\u1ea1i l\u00e0 m\u1ed9t kh\u00e1i ni\u1ec7m c\u01a1 b\u1ea3n trong h\u1ec7 th\u1ed1ng qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u nh\u1eb1m thi\u1ebft l\u1eadp m\u1ed1i quan h\u1ec7 gi\u1eefa hai b\u1ea3ng trong c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7. N\u00f3 \u0111\u1ea3m b\u1ea3o t\u00ednh to\u00e0n v\u1eb9n d\u1eef li\u1ec7u v\u00e0 duy tr\u00ec t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu b\u1eb1ng c\u00e1ch th\u1ef1c thi k\u1ebft n\u1ed1i gi\u1eefa kh\u00f3a ch\u00ednh c\u1ee7a m\u1ed9t b\u1ea3ng v\u00e0 tr\u01b0\u1eddng t\u01b0\u01a1ng \u1ee9ng trong b\u1ea3ng kh\u00e1c. C\u00f4ng c\u1ee5 m\u1ea1nh m\u1ebd n\u00e0y cho ph\u00e9p c\u00e1c nh\u00e0 ph\u00e1t tri\u1ec3n t\u1ea1o ra c\u00e1c li\u00ean k\u1ebft c\u00f3 \u00fd ngh\u0129a gi\u1eefa d\u1eef li\u1ec7u, t\u1ea1o \u0111i\u1ec1u ki\u1ec7n thu\u1eadn l\u1ee3i cho vi\u1ec7c truy xu\u1ea5t v\u00e0 ph\u00e2n t\u00edch d\u1eef li\u1ec7u ph\u1ee9c t\u1ea1p.<\/p>\n<h2>L\u1ecbch s\u1eed ngu\u1ed3n g\u1ed1c c\u1ee7a Kh\u00f3a ngo\u1ea1i v\u00e0 l\u1ea7n \u0111\u1ea7u ti\u00ean nh\u1eafc t\u1edbi n\u00f3<\/h2>\n<p>Kh\u00e1i ni\u1ec7m Kh\u00f3a ngo\u1ea1i l\u1ea7n \u0111\u1ea7u ti\u00ean \u0111\u01b0\u1ee3c EF Codd gi\u1edbi thi\u1ec7u trong b\u00e0i b\u00e1o chuy\u00ean \u0111\u1ec1 c\u1ee7a \u00f4ng c\u00f3 t\u1ef1a \u0111\u1ec1 \u201cM\u00f4 h\u00ecnh d\u1eef li\u1ec7u quan h\u1ec7 cho c\u00e1c ng\u00e2n h\u00e0ng d\u1eef li\u1ec7u chia s\u1ebb l\u1edbn\u201d xu\u1ea5t b\u1ea3n n\u0103m 1970. Codd \u0111\u00e3 \u0111\u1eb7t n\u1ec1n m\u00f3ng cho m\u00f4 h\u00ecnh c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7 v\u00e0 kh\u00f3a ngo\u1ea1i n\u1ed5i l\u00ean nh\u01b0 m\u1ed9t trong nh\u1eefng m\u00f4 h\u00ecnh \u0111\u00f3. c\u00e1c th\u00e0nh ph\u1ea7n quan tr\u1ecdng.<\/p>\n<h2>Th\u00f4ng tin chi ti\u1ebft v\u1ec1 Kh\u00f3a ngo\u1ea1i. M\u1edf r\u1ed9ng ch\u1ee7 \u0111\u1ec1 Kh\u00f3a ngo\u1ea1i<\/h2>\n<p>Kh\u00f3a ngo\u1ea1i \u0111\u00f3ng vai tr\u00f2 l\u00e0 m\u1ed9t kh\u00eda c\u1ea1nh quan tr\u1ecdng trong vi\u1ec7c duy tr\u00ec t\u00ednh to\u00e0n v\u1eb9n d\u1eef li\u1ec7u trong c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7. Khi kh\u00f3a ngo\u1ea1i \u0111\u01b0\u1ee3c thi\u1ebft l\u1eadp gi\u1eefa hai b\u1ea3ng, n\u00f3 \u0111\u1ea3m b\u1ea3o r\u1eb1ng c\u00e1c gi\u00e1 tr\u1ecb trong c\u1ed9t Kh\u00f3a ngo\u00e0i c\u1ee7a m\u1ed9t b\u1ea3ng t\u01b0\u01a1ng \u1ee9ng v\u1edbi c\u00e1c gi\u00e1 tr\u1ecb trong c\u1ed9t kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng kh\u00e1c. B\u1eb1ng c\u00e1ch n\u00e0y, n\u00f3 t\u1ea1o ra m\u1ed1i quan h\u1ec7 cha-con gi\u1eefa c\u00e1c b\u1ea3ng.<\/p>\n<p>M\u1ee5c \u0111\u00edch ch\u00ednh c\u1ee7a kh\u00f3a ngo\u1ea1i nh\u01b0 sau:<\/p>\n<ol>\n<li>\n<p><strong>T\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu:<\/strong> Kh\u00f3a ngo\u1ea1i \u0111\u1ea3m b\u1ea3o r\u1eb1ng d\u1eef li\u1ec7u trong b\u1ea3ng ph\u1ee5 thu\u1ed9c (con) t\u01b0\u01a1ng \u1ee9ng ch\u00ednh x\u00e1c v\u1edbi d\u1eef li\u1ec7u trong b\u1ea3ng \u0111\u01b0\u1ee3c tham chi\u1ebfu (b\u1ea3ng m\u1eb9). N\u00f3 ng\u0103n ch\u1eb7n vi\u1ec7c t\u1ea1o ra c\u00e1c b\u1ea3n ghi m\u1ed3 c\u00f4i v\u00e0 \u0111\u1ea3m b\u1ea3o t\u00ednh nh\u1ea5t qu\u00e1n.<\/p>\n<\/li>\n<li>\n<p><strong>To\u00e0n v\u1eb9n d\u1eef li\u1ec7u:<\/strong> B\u1eb1ng c\u00e1ch th\u1ef1c thi t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu, Kh\u00f3a ngo\u1ea1i ng\u0103n ch\u1eb7n d\u1eef li\u1ec7u kh\u00f4ng h\u1ee3p l\u1ec7 ho\u1eb7c kh\u00f4ng nh\u1ea5t qu\u00e1n \u0111\u01b0\u1ee3c ch\u00e8n v\u00e0o c\u01a1 s\u1edf d\u1eef li\u1ec7u, l\u00e0m gi\u1ea3m s\u1ef1 b\u1ea5t th\u01b0\u1eddng c\u1ee7a d\u1eef li\u1ec7u.<\/p>\n<\/li>\n<li>\n<p><strong>Ph\u1ee5c h\u1ed3i d\u1eef li\u1ec7u:<\/strong> Kh\u00f3a ngo\u1ea1i cho ph\u00e9p nh\u00e0 ph\u00e1t tri\u1ec3n truy xu\u1ea5t d\u1eef li\u1ec7u m\u1ed9t c\u00e1ch hi\u1ec7u qu\u1ea3 b\u1eb1ng c\u00e1ch thi\u1ebft l\u1eadp m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c b\u1ea3ng c\u00f3 li\u00ean quan.<\/p>\n<\/li>\n<li>\n<p><strong>Ho\u1ea1t \u0111\u1ed9ng x\u1ebfp t\u1ea7ng:<\/strong> Kh\u00f3a ngo\u1ea1i c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c \u0111\u1ecbnh c\u1ea5u h\u00ecnh b\u1eb1ng c\u00e1c h\u00e0nh \u0111\u1ed9ng x\u1ebfp t\u1ea7ng, ch\u1eb3ng h\u1ea1n nh\u01b0 CASCADE DELETE ho\u1eb7c CASCADE UPDATE, \u0111\u1ec3 t\u1ef1 \u0111\u1ed9ng truy\u1ec1n c\u00e1c thay \u0111\u1ed5i tr\u00ean c\u00e1c b\u1ea3ng \u0111\u01b0\u1ee3c li\u00ean k\u1ebft.<\/p>\n<\/li>\n<\/ol>\n<h2>C\u1ea5u tr\u00fac b\u00ean trong c\u1ee7a kh\u00f3a ngo\u1ea1i. Kh\u00f3a ngo\u1ea1i ho\u1ea1t \u0111\u1ed9ng nh\u01b0 th\u1ebf n\u00e0o<\/h2>\n<p>Trong n\u1ed9i b\u1ed9, Kh\u00f3a ngo\u1ea1i \u0111\u01b0\u1ee3c tri\u1ec3n khai d\u01b0\u1edbi d\u1ea1ng c\u1ed9t ho\u1eb7c t\u1eadp h\u1ee3p c\u00e1c c\u1ed9t trong b\u1ea3ng con tham chi\u1ebfu kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng cha. Khi m\u1ed9t b\u1ea3n ghi m\u1edbi \u0111\u01b0\u1ee3c ch\u00e8n v\u00e0o b\u1ea3ng con, c\u1ed9t Kh\u00f3a ngo\u00e0i \u0111\u01b0\u1ee3c \u0111i\u1ec1n v\u1edbi gi\u00e1 tr\u1ecb t\u01b0\u01a1ng \u1ee9ng v\u1edbi kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3n ghi li\u00ean quan trong b\u1ea3ng cha. N\u1ebfu b\u1ea3n ghi \u0111\u01b0\u1ee3c tham chi\u1ebfu trong b\u1ea3ng cha b\u1ecb s\u1eeda \u0111\u1ed5i ho\u1eb7c x\u00f3a, Kh\u00f3a ngo\u1ea1i \u0111\u1ea3m b\u1ea3o r\u1eb1ng c\u00e1c h\u00e0nh \u0111\u1ed9ng th\u00edch h\u1ee3p \u0111\u01b0\u1ee3c th\u1ef1c hi\u1ec7n \u0111\u1ec3 duy tr\u00ec t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu.<\/p>\n<p>C\u00fa ph\u00e1p t\u1ea1o kh\u00f3a ngo\u1ea1i th\u01b0\u1eddng li\u00ean quan \u0111\u1ebfn vi\u1ec7c x\u00e1c \u0111\u1ecbnh m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c b\u1ea3ng b\u1eb1ng l\u1ec7nh SQL. V\u00ed d\u1ee5:<\/p>\n<pre><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\"><span>sql<\/span><button class=\"flex ml-auto gap-2\"><svg stroke=\"currentColor\" fill=\"none\" stroke-width=\"2\" viewbox=\"0 0 24 24\" stroke-linecap=\"round\" stroke-linejoin=\"round\" class=\"h-4 w-4\" height=\"1em\" width=\"1em\" ><path d=\"M16 4h2a2 2 0 0 1 2 2v14a2 2 0 0 1-2 2H6a2 2 0 0 1-2-2V6a2 2 0 0 1 2-2h2\"><\/path><rect x=\"8\" y=\"2\" width=\"8\" height=\"4\" rx=\"1\" ry=\"1\"><\/rect><\/svg>Sao ch\u00e9p m\u00e3<\/button><\/div><div class=\"p-4 overflow-y-auto\"><code class=\"!whitespace-pre hljs language-sql\" data-no-translation=\"\"><span class=\"hljs-keyword\">CREATE<\/span> <span class=\"hljs-keyword\">TABLE<\/span> Employees (\n    employee_id <span class=\"hljs-type\">INT<\/span> <span class=\"hljs-keyword\">PRIMARY<\/span> KEY,\n    employee_name <span class=\"hljs-type\">VARCHAR<\/span>(<span class=\"hljs-number\">50<\/span>),\n    department_id <span class=\"hljs-type\">INT<\/span>,\n    <span class=\"hljs-keyword\">FOREIGN<\/span> KEY (department_id) <span class=\"hljs-keyword\">REFERENCES<\/span> Departments(department_id)\n);\n<\/code><\/div><\/div><\/pre>\n<p>Trong v\u00ed d\u1ee5 n\u00e0y, <code data-no-translation=\"\">department_id<\/code> c\u1ed9t trong <code data-no-translation=\"\">Employees<\/code> b\u1ea3ng l\u00e0 kh\u00f3a ngo\u1ea1i tham chi\u1ebfu \u0111\u1ebfn <code data-no-translation=\"\">department_id<\/code> c\u1ed9t trong <code data-no-translation=\"\">Departments<\/code> b\u00e0n.<\/p>\n<h2>Ph\u00e2n t\u00edch c\u00e1c t\u00ednh n\u0103ng ch\u00ednh c\u1ee7a Kh\u00f3a ngo\u1ea1i<\/h2>\n<p>C\u00e1c t\u00ednh n\u0103ng ch\u00ednh c\u1ee7a Kh\u00f3a ngo\u1ea1i bao g\u1ed3m:<\/p>\n<ol>\n<li>\n<p><strong>Thi\u1ebft l\u1eadp m\u1ed1i quan h\u1ec7:<\/strong> Kh\u00f3a ngo\u1ea1i cho ph\u00e9p t\u1ea1o m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c b\u1ea3ng, cho ph\u00e9p bi\u1ec3u di\u1ec5n c\u00e1c li\u00ean k\u1ebft d\u1eef li\u1ec7u ph\u1ee9c t\u1ea1p.<\/p>\n<\/li>\n<li>\n<p><strong>T\u00ednh nh\u1ea5t qu\u00e1n d\u1eef li\u1ec7u:<\/strong> B\u1eb1ng c\u00e1ch th\u1ef1c thi t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu, Kh\u00f3a ngo\u1ea1i duy tr\u00ec t\u00ednh nh\u1ea5t qu\u00e1n c\u1ee7a d\u1eef li\u1ec7u tr\u00ean c\u00e1c b\u1ea3ng \u0111\u01b0\u1ee3c li\u00ean k\u1ebft.<\/p>\n<\/li>\n<li>\n<p><strong>T\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n:<\/strong> Vi\u1ec7c s\u1eed d\u1ee5ng kh\u00f3a ngo\u1ea1i trong truy v\u1ea5n c\u01a1 s\u1edf d\u1eef li\u1ec7u gi\u00fap t\u1ed1i \u01b0u h\u00f3a thao t\u00e1c truy xu\u1ea5t d\u1eef li\u1ec7u, gi\u00fap truy v\u1ea5n nhanh h\u01a1n v\u00e0 hi\u1ec7u qu\u1ea3 h\u01a1n.<\/p>\n<\/li>\n<li>\n<p><strong>Ng\u0103n ch\u1eb7n h\u1ed3 s\u01a1 m\u1ed3 c\u00f4i:<\/strong> Kh\u00f3a ngo\u1ea1i \u0111\u1ea3m b\u1ea3o r\u1eb1ng c\u00e1c b\u1ea3n ghi trong b\u1ea3ng con lu\u00f4n c\u00f3 b\u1ea3n ghi t\u01b0\u01a1ng \u1ee9ng trong b\u1ea3ng cha, ng\u0103n ng\u1eeba c\u00e1c b\u1ea3n ghi m\u1ed3 c\u00f4i.<\/p>\n<\/li>\n<li>\n<p><strong>H\u00e0nh \u0111\u1ed9ng x\u1ebfp t\u1ea7ng:<\/strong> Kh\u1ea3 n\u0103ng \u0111\u1ecbnh c\u1ea5u h\u00ecnh c\u00e1c h\u00e0nh \u0111\u1ed9ng x\u1ebfp t\u1ea7ng gi\u00fap \u0111\u01a1n gi\u1ea3n h\u00f3a qu\u00e1 tr\u00ecnh x\u1eed l\u00fd c\u00e1c thay \u0111\u1ed5i trong d\u1eef li\u1ec7u \u0111\u01b0\u1ee3c li\u00ean k\u1ebft.<\/p>\n<\/li>\n<\/ol>\n<h2>C\u00e1c lo\u1ea1i kh\u00f3a ngo\u1ea1i<\/h2>\n<p>Kh\u00f3a ngo\u1ea1i c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c ph\u00e2n lo\u1ea1i d\u1ef1a tr\u00ean h\u00e0nh vi v\u00e0 r\u00e0ng bu\u1ed9c c\u1ee7a ch\u00fang. C\u00e1c lo\u1ea1i kh\u00f3a ngo\u1ea1i ph\u1ed5 bi\u1ebfn bao g\u1ed3m:<\/p>\n<ol>\n<li>\n<p><strong>Kh\u00f3a ngo\u1ea1i \u0111\u01a1n gi\u1ea3n:<\/strong> M\u1ed9t c\u1ed9t duy nh\u1ea5t tham chi\u1ebfu \u0111\u1ebfn kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng cha.<\/p>\n<\/li>\n<li>\n<p><strong>Kh\u00f3a ngo\u1ea1i t\u1ed5ng h\u1ee3p:<\/strong> Nhi\u1ec1u c\u1ed9t c\u00f9ng tham chi\u1ebfu kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng cha.<\/p>\n<\/li>\n<li>\n<p><strong>T\u1ef1 tham kh\u1ea3o Kh\u00f3a ngo\u1ea1i:<\/strong> M\u1ed9t c\u1ed9t trong b\u1ea3ng tham chi\u1ebfu \u0111\u1ebfn kh\u00f3a ch\u00ednh c\u1ee7a c\u00f9ng m\u1ed9t b\u1ea3ng, t\u1ea1o ra m\u1ed1i quan h\u1ec7 ph\u00e2n c\u1ea5p trong ch\u00ednh b\u1ea3ng \u0111\u00f3.<\/p>\n<\/li>\n<\/ol>\n<h2>C\u00e1ch s\u1eed d\u1ee5ng Kh\u00f3a ngo\u1ea1i, c\u00e1c v\u1ea5n \u0111\u1ec1 v\u00e0 gi\u1ea3i ph\u00e1p li\u00ean quan \u0111\u1ebfn vi\u1ec7c s\u1eed d\u1ee5ng<\/h2>\n<h3>S\u1eed d\u1ee5ng kh\u00f3a ngo\u1ea1i:<\/h3>\n<ul>\n<li>X\u00e1c \u0111\u1ecbnh m\u1ed1i quan h\u1ec7: S\u1eed d\u1ee5ng Kh\u00f3a ngo\u1ea1i \u0111\u1ec3 thi\u1ebft l\u1eadp m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c b\u1ea3ng bi\u1ec3u di\u1ec5n d\u1eef li\u1ec7u li\u00ean quan.<\/li>\n<li>\u0110\u1ea3m b\u1ea3o t\u00ednh to\u00e0n v\u1eb9n d\u1eef li\u1ec7u: Kh\u00f3a ngo\u1ea1i th\u1ef1c thi t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu, \u0111\u1ea3m b\u1ea3o t\u00ednh nh\u1ea5t qu\u00e1n c\u1ee7a d\u1eef li\u1ec7u.<\/li>\n<\/ul>\n<h3>V\u1ea5n \u0111\u1ec1 v\u00e0 gi\u1ea3i ph\u00e1p:<\/h3>\n<ol>\n<li>\n<p><strong>C\u00e1c b\u1ea5t th\u01b0\u1eddng khi ch\u00e8n\/x\u00f3a:<\/strong> Vi\u1ec7c s\u1eed d\u1ee5ng kh\u00f3a ngo\u1ea1i kh\u00f4ng \u0111\u00fang c\u00e1ch c\u00f3 th\u1ec3 d\u1eabn \u0111\u1ebfn s\u1ef1 b\u1ea5t th\u01b0\u1eddng khi ch\u00e8n v\u00e0 x\u00f3a. \u0110\u1ec3 ng\u0103n ch\u1eb7n \u0111i\u1ec1u n\u00e0y, h\u00e3y tr\u00e1nh tham chi\u1ebfu v\u00f2ng tr\u00f2n v\u00e0 s\u1eed d\u1ee5ng c\u00e1c h\u00e0nh \u0111\u1ed9ng x\u1ebfp t\u1ea7ng m\u1ed9t c\u00e1ch kh\u00f4n ngoan.<\/p>\n<\/li>\n<li>\n<p><strong>T\u00e1c \u0111\u1ed9ng hi\u1ec7u su\u1ea5t:<\/strong> Vi\u1ec7c l\u1ea1m d\u1ee5ng kh\u00f3a ngo\u1ea1i ho\u1eb7c x\u00e1c \u0111\u1ecbnh ch\u00fang tr\u00ean c\u00e1c c\u1ed9t \u0111\u01b0\u1ee3c c\u1eadp nh\u1eadt th\u01b0\u1eddng xuy\u00ean c\u00f3 th\u1ec3 \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn hi\u1ec7u su\u1ea5t c\u01a1 s\u1edf d\u1eef li\u1ec7u. S\u1eed d\u1ee5ng kh\u00f3a ngo\u1ea1i m\u1ed9t c\u00e1ch th\u1eadn tr\u1ecdng v\u00e0 xem x\u00e9t vi\u1ec7c l\u1eadp ch\u1ec9 m\u1ee5c.<\/p>\n<\/li>\n<li>\n<p><strong>X\u1eed l\u00fd gi\u00e1 tr\u1ecb NULL:<\/strong> Khi x\u1eed l\u00fd c\u00e1c kh\u00f3a ngo\u1ea1i c\u00f3 th\u1ec3 r\u1ed7ng, c\u1ea7n \u0111\u1eb7c bi\u1ec7t ch\u00fa \u00fd x\u1eed l\u00fd c\u00e1c gi\u00e1 tr\u1ecb NULL m\u1ed9t c\u00e1ch th\u00edch h\u1ee3p.<\/p>\n<\/li>\n<\/ol>\n<h2>C\u00e1c \u0111\u1eb7c \u0111i\u1ec3m ch\u00ednh v\u00e0 so s\u00e1nh kh\u00e1c v\u1edbi c\u00e1c thu\u1eadt ng\u1eef t\u01b0\u01a1ng t\u1ef1 d\u01b0\u1edbi d\u1ea1ng b\u1ea3ng v\u00e0 danh s\u00e1ch<\/h2>\n<table>\n<thead>\n<tr>\n<th><strong>\u0111\u1eb7c tr\u01b0ng<\/strong><\/th>\n<th><strong>Kh\u00f3a ngo\u1ea1i<\/strong><\/th>\n<th><strong>Kh\u00f3a ch\u00ednh<\/strong><\/th>\n<th><strong>Ch\u00eca kh\u00f3a duy nh\u1ea5t<\/strong><\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>M\u1ee5c \u0111\u00edch<\/td>\n<td>Thi\u1ebft l\u1eadp m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c b\u1ea3ng b\u1eb1ng c\u00e1ch tham chi\u1ebfu kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng kh\u00e1c.<\/td>\n<td>X\u00e1c \u0111\u1ecbnh duy nh\u1ea5t t\u1eebng b\u1ea3n ghi trong b\u1ea3ng.<\/td>\n<td>\u0110\u1ea3m b\u1ea3o t\u00ednh duy nh\u1ea5t c\u1ee7a c\u00e1c gi\u00e1 tr\u1ecb trong m\u1ed9t c\u1ed9t.<\/td>\n<\/tr>\n<tr>\n<td>T\u00ednh duy nh\u1ea5t<\/td>\n<td>Kh\u00f4ng nh\u1ea5t thi\u1ebft ph\u1ea3i l\u00e0 duy nh\u1ea5t; nhi\u1ec1u h\u00e0ng c\u00f3 th\u1ec3 c\u00f3 c\u00f9ng gi\u00e1 tr\u1ecb kh\u00f3a ngo\u1ea1i.<\/td>\n<td>\u0110\u1ed9c nh\u1ea5t; m\u1ed7i h\u00e0ng c\u00f3 m\u1ed9t gi\u00e1 tr\u1ecb kh\u00f3a ch\u00ednh ri\u00eang bi\u1ec7t.<\/td>\n<td>\u0110\u1ed9c nh\u1ea5t; m\u1ed7i h\u00e0ng c\u00f3 m\u1ed9t gi\u00e1 tr\u1ecb kh\u00f3a ri\u00eang bi\u1ec7t.<\/td>\n<\/tr>\n<tr>\n<td>Gi\u00e1 tr\u1ecb r\u1ed7ng<\/td>\n<td>Cho ph\u00e9p c\u00e1c gi\u00e1 tr\u1ecb null th\u1ec3 hi\u1ec7n c\u00e1c m\u1ed1i quan h\u1ec7 b\u1ecb thi\u1ebfu.<\/td>\n<td>Kh\u00f4ng cho ph\u00e9p gi\u00e1 tr\u1ecb null; ph\u1ea3i c\u00f3 m\u1eb7t cho m\u1ed7i h\u00e0ng.<\/td>\n<td>Cho ph\u00e9p gi\u00e1 tr\u1ecb null, nh\u01b0ng ch\u1ec9 m\u1ed9t h\u00e0ng c\u00f3 th\u1ec3 c\u00f3 gi\u00e1 tr\u1ecb kh\u00f3a null.<\/td>\n<\/tr>\n<tr>\n<td>S\u1ed1 l\u1ea7n xu\u1ea5t hi\u1ec7n<\/td>\n<td>Nhi\u1ec1u kh\u00f3a ngo\u1ea1i c\u00f3 th\u1ec3 t\u1ed3n t\u1ea1i trong m\u1ed9t b\u1ea3ng.<\/td>\n<td>Ch\u1ec9 c\u00f3 m\u1ed9t kh\u00f3a ch\u00ednh c\u00f3 th\u1ec3 t\u1ed3n t\u1ea1i trong m\u1ed9t b\u1ea3ng.<\/td>\n<td>Ch\u1ec9 c\u00f3 m\u1ed9t kh\u00f3a duy nh\u1ea5t c\u00f3 th\u1ec3 t\u1ed3n t\u1ea1i trong m\u1ed9t b\u1ea3ng.<\/td>\n<\/tr>\n<tr>\n<td>M\u1ed1i quan h\u1ec7 v\u1edbi d\u1eef li\u1ec7u<\/td>\n<td>\u0110\u01b0\u1ee3c li\u00ean k\u1ebft v\u1edbi kh\u00f3a ch\u00ednh trong m\u1ed9t b\u1ea3ng kh\u00e1c.<\/td>\n<td>Li\u00ean k\u1ebft v\u1edbi d\u1eef li\u1ec7u trong c\u00f9ng m\u1ed9t b\u1ea3ng.<\/td>\n<td>Li\u00ean k\u1ebft v\u1edbi d\u1eef li\u1ec7u trong c\u00f9ng b\u1ea3ng ho\u1eb7c b\u1ea3ng kh\u00e1c.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2>Tri\u1ec3n v\u1ecdng v\u00e0 c\u00f4ng ngh\u1ec7 t\u01b0\u01a1ng lai li\u00ean quan \u0111\u1ebfn Kh\u00f3a ngo\u1ea1i<\/h2>\n<p>T\u01b0\u01a1ng lai c\u1ee7a Kh\u00f3a ngo\u1ea1i n\u1eb1m \u1edf vai tr\u00f2 ti\u1ebfp t\u1ee5c c\u1ee7a ch\u00fang l\u00e0 n\u1ec1n t\u1ea3ng cho t\u00ednh to\u00e0n v\u1eb9n d\u1eef li\u1ec7u trong c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7. Khi c\u00f4ng ngh\u1ec7 ph\u00e1t tri\u1ec3n, h\u1ec7 th\u1ed1ng qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u c\u00f3 th\u1ec3 \u0111\u01b0a ra c\u00e1c c\u1ea3i ti\u1ebfn v\u00e0 t\u1ed1i \u01b0u h\u00f3a \u0111\u1ec3 n\u00e2ng cao hi\u1ec7u su\u1ea5t v\u00e0 kh\u1ea3 n\u0103ng c\u1ee7a Kh\u00f3a ngo\u1ea1i. M\u1ed9t s\u1ed1 ph\u00e1t tri\u1ec3n ti\u1ec1m n\u0103ng trong t\u01b0\u01a1ng lai bao g\u1ed3m:<\/p>\n<ol>\n<li>\n<p><strong>L\u1eadp ch\u1ec9 m\u1ee5c t\u1ef1 \u0111\u1ed9ng:<\/strong> C\u00e1c thu\u1eadt to\u00e1n n\u00e2ng cao c\u00f3 th\u1ec3 t\u1ef1 \u0111\u1ed9ng h\u00f3a vi\u1ec7c nh\u1eadn d\u1ea1ng v\u00e0 t\u1ea1o ch\u1ec9 m\u1ee5c tr\u00ean c\u00e1c c\u1ed9t Kh\u00f3a ngo\u1ea1i, t\u1ed1i \u01b0u h\u00f3a hi\u1ec7u su\u1ea5t truy v\u1ea5n.<\/p>\n<\/li>\n<li>\n<p><strong>Sao ch\u00e9p v\u00e0 ph\u00e2n chia hi\u1ec7u qu\u1ea3:<\/strong> Nh\u1eefng \u0111\u1ed5i m\u1edbi c\u00f3 th\u1ec3 cho ph\u00e9p c\u00e1c chi\u1ebfn l\u01b0\u1ee3c sao ch\u00e9p v\u00e0 ph\u00e2n chia d\u1eef li\u1ec7u hi\u1ec7u qu\u1ea3 h\u01a1n li\u00ean quan \u0111\u1ebfn c\u00e1c b\u1ea3ng c\u00f3 m\u1ed1i quan h\u1ec7 kh\u00f3a ngo\u00e0i.<\/p>\n<\/li>\n<li>\n<p><strong>C\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u1ed3 th\u1ecb:<\/strong> S\u1ef1 gia t\u0103ng c\u1ee7a c\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u1ed3 th\u1ecb c\u00f3 th\u1ec3 gi\u1edbi thi\u1ec7u nh\u1eefng c\u00e1ch m\u1edbi \u0111\u1ec3 x\u1eed l\u00fd m\u1ed1i quan h\u1ec7 gi\u1eefa d\u1eef li\u1ec7u, c\u00f3 kh\u1ea3 n\u0103ng x\u00e1c \u0111\u1ecbnh l\u1ea1i kh\u00e1i ni\u1ec7m Kh\u00f3a ngo\u1ea1i trong b\u1ed1i c\u1ea3nh kh\u00f4ng quan h\u1ec7.<\/p>\n<\/li>\n<\/ol>\n<h2>C\u00e1ch s\u1eed d\u1ee5ng ho\u1eb7c li\u00ean k\u1ebft m\u00e1y ch\u1ee7 proxy v\u1edbi Kh\u00f3a ngo\u1ea1i<\/h2>\n<p>Trong b\u1ed1i c\u1ea3nh nh\u00e0 cung c\u1ea5p m\u00e1y ch\u1ee7 proxy nh\u01b0 OneProxy (oneproxy.pro), Kh\u00f3a ngo\u1ea1i c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng n\u1ed9i b\u1ed9 \u0111\u1ec3 qu\u1ea3n l\u00fd m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c b\u1ea3ng c\u01a1 s\u1edf d\u1eef li\u1ec7u kh\u00e1c nhau ch\u1ee9a th\u00f4ng tin v\u1ec1 t\u00e0i kho\u1ea3n ng\u01b0\u1eddi d\u00f9ng, g\u00f3i \u0111\u0103ng k\u00fd, chi ti\u1ebft thanh to\u00e1n v\u00e0 v\u1ecb tr\u00ed m\u00e1y ch\u1ee7. V\u00ed d\u1ee5:<\/p>\n<ul>\n<li>Kh\u00f3a ngo\u1ea1i c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 li\u00ean k\u1ebft th\u00f4ng tin t\u00e0i kho\u1ea3n c\u1ee7a ng\u01b0\u1eddi d\u00f9ng v\u1edbi g\u00f3i \u0111\u0103ng k\u00fd t\u01b0\u01a1ng \u1ee9ng c\u1ee7a h\u1ecd.<\/li>\n<li>M\u1ed9t kh\u00f3a ngo\u1ea1i kh\u00e1c c\u00f3 th\u1ec3 thi\u1ebft l\u1eadp m\u1ed1i quan h\u1ec7 gi\u1eefa d\u1eef li\u1ec7u ng\u01b0\u1eddi d\u00f9ng v\u00e0 v\u1ecb tr\u00ed m\u00e1y ch\u1ee7 \u0111\u00e3 ch\u1ecdn c\u1ee7a h\u1ecd.<\/li>\n<\/ul>\n<p>B\u1eb1ng c\u00e1ch s\u1eed d\u1ee5ng Kh\u00f3a ngo\u1ea1i, OneProxy c\u00f3 th\u1ec3 \u0111\u1ea3m b\u1ea3o t\u00ednh nh\u1ea5t qu\u00e1n c\u1ee7a d\u1eef li\u1ec7u, t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu v\u00e0 truy xu\u1ea5t d\u1eef li\u1ec7u hi\u1ec7u qu\u1ea3 trong c\u01a1 s\u1edf d\u1eef li\u1ec7u c\u1ee7a h\u1ecd, g\u00f3p ph\u1ea7n v\u00e0o ho\u1ea1t \u0111\u1ed9ng li\u1ec1n m\u1ea1ch c\u1ee7a d\u1ecbch v\u1ee5 proxy c\u1ee7a h\u1ecd.<\/p>\n<h2>Li\u00ean k\u1ebft li\u00ean quan<\/h2>\n<p>\u0110\u1ec3 bi\u1ebft th\u00eam th\u00f4ng tin v\u1ec1 Kh\u00f3a ngo\u1ea1i, b\u1ea1n c\u00f3 th\u1ec3 kh\u00e1m ph\u00e1 c\u00e1c t\u00e0i nguy\u00ean sau:<\/p>\n<ul>\n<li><a href=\"https:\/\/www.w3schools.com\/sql\/sql_foreignkey.asp\" target=\"_new\" rel=\"noopener nofollow\">Gi\u1edbi thi\u1ec7u v\u1ec1 kh\u00f3a ngo\u1ea1i trong SQL<\/a><\/li>\n<li><a href=\"https:\/\/www.sqlshack.com\/understanding-the-sql-server-foreign-key\/\" target=\"_new\" rel=\"noopener nofollow\">T\u00ecm hi\u1ec3u kh\u00f3a ngo\u1ea1i trong qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u<\/a><\/li>\n<li><a href=\"https:\/\/www.ibm.com\/docs\/en\/informix-servers\/12.10?topic=keys-using-foreign-keys-data-integrity\" target=\"_new\" rel=\"noopener nofollow\">S\u1eed d\u1ee5ng kh\u00f3a ngo\u1ea1i \u0111\u1ec3 to\u00e0n v\u1eb9n d\u1eef li\u1ec7u<\/a><\/li>\n<\/ul>\n<p>T\u00f3m l\u1ea1i, Kh\u00f3a ngo\u1ea1i l\u00e0 th\u00e0nh ph\u1ea7n quan tr\u1ecdng c\u1ee7a c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7, cho ph\u00e9p thi\u1ebft l\u1eadp m\u1ed1i quan h\u1ec7 gi\u1eefa d\u1eef li\u1ec7u v\u00e0 \u0111\u1ea3m b\u1ea3o t\u00ednh to\u00e0n v\u1eb9n c\u1ee7a d\u1eef li\u1ec7u. Khi c\u01a1 s\u1edf d\u1eef li\u1ec7u v\u00e0 c\u00f4ng ngh\u1ec7 ti\u1ebfp t\u1ee5c ph\u00e1t tri\u1ec3n, Kh\u00f3a ngo\u1ea1i s\u1ebd v\u1eabn l\u00e0 m\u1ed9t c\u00f4ng c\u1ee5 thi\u1ebft y\u1ebfu \u0111\u1ec3 qu\u1ea3n l\u00fd v\u00e0 t\u1ed5 ch\u1ee9c c\u00e1c c\u1ea5u tr\u00fac d\u1eef li\u1ec7u ph\u1ee9c t\u1ea1p trong nhi\u1ec1u \u1ee9ng d\u1ee5ng kh\u00e1c nhau, bao g\u1ed3m c\u1ea3 c\u00e1c nh\u00e0 cung c\u1ea5p m\u00e1y ch\u1ee7 proxy nh\u01b0 OneProxy.<\/p>","protected":false},"featured_media":468433,"menu_order":0,"template":"","meta":{"_acf_changed":false,"content-type":"","inline_featured_image":false,"footnotes":""},"class_list":["post-477274","wiki","type-wiki","status-publish","has-post-thumbnail","hentry"],"acf":{"faq_title":"Frequently Asked Questions about <mark>Foreign Key: A Comprehensive Guide<\/mark>","faq_items":[{"question":"What is a Foreign key and why is it important in databases?","answer":"<p>A Foreign key is a crucial concept in database management systems that establishes a relationship between two tables. It ensures data integrity by linking the primary key of one table to a corresponding field in another table. This linkage enables meaningful associations between data, facilitates data retrieval, and prevents inconsistent or invalid data from being inserted.<\/p>"},{"question":"Who introduced the concept of Foreign keys?","answer":"<p>The concept of Foreign keys was first introduced by E.F. Codd in his seminal paper titled \"A Relational Model of Data for Large Shared Data Banks\" published in 1970. Codd's work laid the foundation for the relational database model, and Foreign keys emerged as a key component.<\/p>"},{"question":"How does a Foreign key work internally?","answer":"<p>Internally, a Foreign key is implemented as a column or set of columns in the child table that references the primary key of the parent table. When new data is inserted into the child table, the Foreign key column is populated with a value corresponding to the primary key of the related record in the parent table. If the referenced record in the parent table is modified or deleted, the Foreign key ensures that appropriate actions are taken to maintain referential integrity.<\/p>"},{"question":"What are the main features of Foreign keys?","answer":"<p>The key features of Foreign keys include establishing data relationships, ensuring data consistency, optimizing query performance, and facilitating cascading actions for data changes across linked tables.<\/p>"},{"question":"What are the different types of Foreign keys?","answer":"<p>Foreign keys can be classified into three main types: Simple Foreign keys (with a single column), Composite Foreign keys (with multiple columns), and Self-referencing Foreign keys (where a column in a table references the primary key of the same table).<\/p>"},{"question":"How can Foreign keys be used, and what are the potential issues?","answer":"<p>Foreign keys are used to establish meaningful relationships between tables and ensure data integrity. However, improper use can lead to insertion and deletion anomalies and impact database performance. Care should be taken when handling NULL values in Foreign keys, and circular references should be avoided.<\/p>"},{"question":"How do Foreign keys compare to Primary keys and Unique keys?","answer":"<p>Foreign keys establish relationships between tables, Primary keys uniquely identify records in a table, and Unique keys ensure the uniqueness of values in a column. Each has its own distinct purpose and characteristics.<\/p>"},{"question":"What are the future perspectives of Foreign keys in databases?","answer":"<p>In the future, advancements in database technology may introduce automated indexing, more efficient replication, and the use of Foreign keys in non-relational contexts like graph databases.<\/p>"},{"question":"How are proxy servers associated with Foreign keys?","answer":"<p>Proxy server providers like OneProxy may use Foreign keys internally to manage relationships between various database tables containing user accounts, subscription plans, billing details, and server locations. This ensures seamless operation and data integrity in their proxy services.<\/p>"}]},"_links":{"self":[{"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/wiki\/477274","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/wiki"}],"about":[{"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/types\/wiki"}],"version-history":[{"count":0,"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/wiki\/477274\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/media\/468433"}],"wp:attachment":[{"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/media?parent=477274"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}