{"id":476178,"date":"2023-08-09T07:26:52","date_gmt":"2023-08-09T07:26:52","guid":{"rendered":""},"modified":"2023-09-05T11:12:10","modified_gmt":"2023-09-05T11:12:10","slug":"cardinality-sql","status":"publish","type":"wiki","link":"https:\/\/oneproxy.pro\/vn\/wiki\/cardinality-sql\/","title":{"rendered":"S\u1ed1 l\u01b0\u1ee3ng (SQL)"},"content":{"rendered":"<p>S\u1ed1 l\u01b0\u1ee3ng trong SQL \u0111\u1ec1 c\u1eadp \u0111\u1ebfn s\u1ed1 l\u01b0\u1ee3ng gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t trong m\u1ed9t c\u1ed9t ho\u1eb7c ch\u1ec9 m\u1ee5c c\u1ee7a b\u1ea3ng c\u01a1 s\u1edf d\u1eef li\u1ec7u. N\u00f3 \u0111\u00f3ng m\u1ed9t vai tr\u00f2 quan tr\u1ecdng trong vi\u1ec7c t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n v\u00e0 \u0111i\u1ec1u ch\u1ec9nh hi\u1ec7u su\u1ea5t, v\u00ec n\u00f3 cung c\u1ea5p th\u00f4ng tin chuy\u00ean s\u00e2u v\u1ec1 ph\u00e2n ph\u1ed1i d\u1eef li\u1ec7u v\u00e0 gi\u00fap c\u00f4ng c\u1ee5 c\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u01b0a ra quy\u1ebft \u0111\u1ecbnh s\u00e1ng su\u1ed1t khi t\u1ea1o k\u1ebf ho\u1ea1ch th\u1ef1c hi\u1ec7n. Cardinality l\u00e0 m\u1ed9t kh\u00e1i ni\u1ec7m c\u01a1 b\u1ea3n trong l\u0129nh v\u1ef1c c\u01a1 s\u1edf d\u1eef li\u1ec7u v\u00e0 \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng r\u1ed9ng r\u00e3i trong c\u00e1c h\u1ec7 th\u1ed1ng qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u (DBMS) kh\u00e1c nhau.<\/p>\n<h2>L\u1ecbch s\u1eed ngu\u1ed3n g\u1ed1c c\u1ee7a Cardinality (SQL) v\u00e0 l\u1ea7n \u0111\u1ea7u ti\u00ean \u0111\u1ec1 c\u1eadp \u0111\u1ebfn n\u00f3<\/h2>\n<p>Kh\u00e1i ni\u1ec7m v\u1ec1 Cardinality trong SQL c\u00f3 th\u1ec3 b\u1eaft ngu\u1ed3n t\u1eeb nh\u1eefng ng\u00e0y \u0111\u1ea7u c\u1ee7a c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7. M\u00f4 h\u00ecnh quan h\u1ec7 \u0111\u01b0\u1ee3c Ti\u1ebfn s\u0129 EF Codd gi\u1edbi thi\u1ec7u trong b\u00e0i b\u00e1o \u0111\u1ed9t ph\u00e1 \u201cM\u00f4 h\u00ecnh d\u1eef li\u1ec7u quan h\u1ec7 cho c\u00e1c ng\u00e2n h\u00e0ng d\u1eef li\u1ec7u d\u00f9ng chung l\u1edbn\u201d xu\u1ea5t b\u1ea3n n\u0103m 1970. Trong b\u00e0i b\u00e1o n\u00e0y, Codd tr\u00ecnh b\u00e0y \u00fd t\u01b0\u1edfng bi\u1ec3u di\u1ec5n d\u1eef li\u1ec7u trong c\u00e1c b\u1ea3ng theo h\u00e0ng v\u00e0 c\u1ed9t, c\u00f9ng v\u1edbi m\u1ed9t t\u1eadp h\u1ee3p c\u00e1c ph\u00e9p to\u00e1n \u0111\u1ec3 thao t\u00e1c d\u1eef li\u1ec7u.<\/p>\n<p>Thu\u1eadt ng\u1eef \u201cCardinality\u201d sau \u0111\u00f3 \u0111\u00e3 \u0111\u01b0\u1ee3c ph\u1ed5 bi\u1ebfn r\u1ed9ng r\u00e3i khi c\u00e1c h\u1ec7 th\u1ed1ng qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7 ph\u00e1t tri\u1ec3n v\u00e0 tr\u01b0\u1edfng th\u00e0nh. N\u00f3 tr\u1edf n\u00ean n\u1ed5i b\u1eadt do t\u1ea7m quan tr\u1ecdng c\u1ee7a n\u00f3 trong vi\u1ec7c t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n, trong \u0111\u00f3 vi\u1ec7c \u01b0\u1edbc t\u00ednh s\u1ed1 l\u01b0\u1ee3ng h\u00e0ng s\u1ebd \u0111\u01b0\u1ee3c tr\u1ea3 v\u1ec1 t\u1eeb m\u1ed9t truy v\u1ea5n \u0111\u1ec3 ch\u1ecdn ph\u01b0\u01a1ng \u00e1n th\u1ef1c hi\u1ec7n hi\u1ec7u qu\u1ea3 nh\u1ea5t l\u00e0 \u0111i\u1ec1u c\u1ea7n thi\u1ebft.<\/p>\n<h2>Th\u00f4ng tin chi ti\u1ebft v\u1ec1 Cardinality (SQL)<\/h2>\n<p>Trong ng\u1eef c\u1ea3nh c\u1ee7a c\u01a1 s\u1edf d\u1eef li\u1ec7u SQL, Cardinality \u0111\u1ec1 c\u1eadp \u0111\u1ebfn s\u1ed1 l\u01b0\u1ee3ng gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t c\u00f3 trong m\u1ed9t c\u1ed9t ho\u1eb7c m\u1ed9t ch\u1ec9 m\u1ee5c. N\u00f3 cung c\u1ea5p th\u00f4ng tin th\u1ed1ng k\u00ea v\u1ec1 vi\u1ec7c ph\u00e2n ph\u1ed1i d\u1eef li\u1ec7u trong b\u1ea3ng, gi\u00fap tr\u00ecnh t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n x\u00e1c \u0111\u1ecbnh c\u00e1ch hi\u1ec7u qu\u1ea3 nh\u1ea5t \u0111\u1ec3 x\u1eed l\u00fd truy v\u1ea5n.<\/p>\n<h2>C\u1ea5u tr\u00fac b\u00ean trong c\u1ee7a Cardinality (SQL) v\u00e0 c\u00e1ch th\u1ee9c ho\u1ea1t \u0111\u1ed9ng<\/h2>\n<p>C\u1ea5u tr\u00fac b\u00ean trong c\u1ee7a Cardinality \u0111\u01b0\u1ee3c duy tr\u00ec trong s\u1ed1 li\u1ec7u th\u1ed1ng k\u00ea c\u01a1 s\u1edf d\u1eef li\u1ec7u. DBMS l\u01b0u tr\u1eef s\u1ed1 li\u1ec7u th\u1ed1ng k\u00ea v\u1ec1 c\u00e1c b\u1ea3ng v\u00e0 ch\u1ec9 m\u1ee5c, bao g\u1ed3m th\u00f4ng tin v\u1ec1 s\u1ed1 l\u01b0\u1ee3ng h\u00e0ng, gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t v\u00e0 ph\u00e2n b\u1ed5 d\u1eef li\u1ec7u. Khi m\u1ed9t truy v\u1ea5n \u0111\u01b0\u1ee3c th\u1ef1c thi, tr\u00ecnh t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n s\u1ebd s\u1eed d\u1ee5ng c\u00e1c th\u1ed1ng k\u00ea n\u00e0y \u0111\u1ec3 \u01b0\u1edbc t\u00ednh S\u1ed1 l\u01b0\u1ee3ng v\u00e0 ch\u1ecdn k\u1ebf ho\u1ea1ch th\u1ef1c hi\u1ec7n truy v\u1ea5n t\u1ed1i \u01b0u.<\/p>\n<p>H\u1ec7 th\u1ed1ng qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u c\u00f3 th\u1ec3 s\u1eed d\u1ee5ng nhi\u1ec1u thu\u1eadt to\u00e1n v\u00e0 c\u1ea5u tr\u00fac d\u1eef li\u1ec7u kh\u00e1c nhau \u0111\u1ec3 theo d\u00f5i Cardinality m\u1ed9t c\u00e1ch hi\u1ec7u qu\u1ea3. C\u00e1c c\u1ea5u tr\u00fac n\u00e0y \u0111\u01b0\u1ee3c c\u1eadp nh\u1eadt \u0111\u1ecbnh k\u1ef3 ho\u1eb7c theo y\u00eau c\u1ea7u khi d\u1eef li\u1ec7u thay \u0111\u1ed5i trong c\u01a1 s\u1edf d\u1eef li\u1ec7u.<\/p>\n<h2>Ph\u00e2n t\u00edch c\u00e1c t\u00ednh n\u0103ng ch\u00ednh c\u1ee7a Cardinality (SQL)<\/h2>\n<p>C\u00e1c t\u00ednh n\u0103ng ch\u00ednh c\u1ee7a Cardinality trong SQL bao g\u1ed3m:<\/p>\n<ol>\n<li>\n<p><strong>T\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n:<\/strong> S\u1ed1 l\u01b0\u1ee3ng l\u00e0 m\u1ed9t y\u1ebfu t\u1ed1 quan tr\u1ecdng trong vi\u1ec7c x\u00e1c \u0111\u1ecbnh k\u1ebf ho\u1ea1ch th\u1ef1c hi\u1ec7n cho m\u1ed9t truy v\u1ea5n. S\u1ed1 l\u01b0\u1ee3ng cao h\u01a1n th\u01b0\u1eddng d\u1eabn \u0111\u1ebfn c\u00e1c ch\u1ec9 m\u1ee5c c\u00f3 ch\u1ecdn l\u1ecdc h\u01a1n, d\u1eabn \u0111\u1ebfn vi\u1ec7c th\u1ef1c thi truy v\u1ea5n nhanh h\u01a1n.<\/p>\n<\/li>\n<li>\n<p><strong>Ph\u00e2n t\u00edch ph\u00e2n ph\u1ed1i d\u1eef li\u1ec7u:<\/strong> Cardinality cung c\u1ea5p th\u00f4ng tin chi ti\u1ebft v\u1ec1 vi\u1ec7c ph\u00e2n ph\u1ed1i c\u00e1c gi\u00e1 tr\u1ecb d\u1eef li\u1ec7u trong m\u1ed9t c\u1ed9t. N\u00f3 gi\u00fap x\u00e1c \u0111\u1ecbnh c\u00e1c v\u1ea5n \u0111\u1ec1 ti\u1ec1m \u1ea9n v\u1ec1 ch\u1ea5t l\u01b0\u1ee3ng d\u1eef li\u1ec7u, ch\u1eb3ng h\u1ea1n nh\u01b0 d\u1eef li\u1ec7u b\u1ecb sai l\u1ec7ch ho\u1eb7c c\u00e1c m\u1ee5c nh\u1eadp tr\u00f9ng l\u1eb7p.<\/p>\n<\/li>\n<li>\n<p><strong>Tham gia t\u1ed1i \u01b0u h\u00f3a:<\/strong> Cardinality \u0111\u00f3ng m\u1ed9t vai tr\u00f2 quan tr\u1ecdng trong vi\u1ec7c t\u1ed1i \u01b0u h\u00f3a c\u00e1c ho\u1ea1t \u0111\u1ed9ng tham gia. Tr\u00ecnh t\u1ed1i \u01b0u h\u00f3a c\u01a1 s\u1edf d\u1eef li\u1ec7u s\u1eed d\u1ee5ng Cardinality c\u1ee7a c\u00e1c c\u1ed9t \u0111\u00e3 n\u1ed1i \u0111\u1ec3 ch\u1ecdn chi\u1ebfn l\u01b0\u1ee3c n\u1ed1i hi\u1ec7u qu\u1ea3 nh\u1ea5t, nh\u01b0 n\u1ed1i v\u00f2ng l\u1eb7p l\u1ed3ng nhau, n\u1ed1i b\u0103m ho\u1eb7c n\u1ed1i h\u1ee3p nh\u1ea5t.<\/p>\n<\/li>\n<li>\n<p><strong>Thi\u1ebft k\u1ebf ch\u1ec9 m\u1ee5c:<\/strong> Cardinality \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn hi\u1ec7u qu\u1ea3 c\u1ee7a c\u00e1c ch\u1ec9 m\u1ee5c c\u01a1 s\u1edf d\u1eef li\u1ec7u. C\u00e1c c\u1ed9t Cardinality th\u1ea5p l\u00e0 \u1ee9ng c\u1eed vi\u00ean k\u00e9m cho vi\u1ec7c l\u1eadp ch\u1ec9 m\u1ee5c v\u00ec ch\u00fang kh\u00f4ng mang l\u1ea1i nhi\u1ec1u t\u00ednh ch\u1ecdn l\u1ecdc, trong khi c\u00e1c c\u1ed9t Cardinality cao l\u00e0 \u1ee9ng c\u1eed vi\u00ean t\u1ed1t h\u01a1n cho vi\u1ec7c l\u1eadp ch\u1ec9 m\u1ee5c.<\/p>\n<\/li>\n<\/ol>\n<h2>C\u00e1c lo\u1ea1i s\u1ed1 l\u01b0\u1ee3ng (SQL)<\/h2>\n<p>C\u00f3 ba lo\u1ea1i Cardinality ch\u00ednh:<\/p>\n<ol>\n<li>\n<p><strong>S\u1ed1 l\u01b0\u1ee3ng th\u1ea5p:<\/strong> M\u1ed9t c\u1ed9t c\u00f3 s\u1ed1 l\u01b0\u1ee3ng th\u1ea5p c\u00f3 m\u1ed9t s\u1ed1 l\u01b0\u1ee3ng nh\u1ecf c\u00e1c gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t so v\u1edbi t\u1ed5ng s\u1ed1 h\u00e0ng trong b\u1ea3ng. C\u00e1c v\u00ed d\u1ee5 ph\u1ed5 bi\u1ebfn bao g\u1ed3m c\u00e1c c\u1ed9t gi\u1edbi t\u00ednh ho\u1eb7c qu\u1ed1c gia, th\u01b0\u1eddng ch\u1ec9 c\u00f3 m\u1ed9t v\u00e0i gi\u00e1 tr\u1ecb duy nh\u1ea5t \u0111\u01b0\u1ee3c l\u1eb7p l\u1ea1i tr\u00ean nhi\u1ec1u h\u00e0ng.<\/p>\n<\/li>\n<li>\n<p><strong>Nhi\u1ec7t \u0111\u1ed9 cao:<\/strong> M\u1ed9t c\u1ed9t c\u00f3 Cardinality cao c\u00f3 s\u1ed1 l\u01b0\u1ee3ng l\u1edbn c\u00e1c gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t so v\u1edbi t\u1ed5ng s\u1ed1 h\u00e0ng trong b\u1ea3ng. V\u00ed d\u1ee5: kh\u00f3a ch\u00ednh ho\u1eb7c c\u1ed9t \u0111\u1ecbnh danh duy nh\u1ea5t c\u00f3 xu h\u01b0\u1edbng c\u00f3 s\u1ed1 l\u01b0\u1ee3ng cao v\u00ec m\u1ed7i h\u00e0ng c\u00f3 m\u1ed9t gi\u00e1 tr\u1ecb duy nh\u1ea5t.<\/p>\n<\/li>\n<li>\n<p><strong>S\u1ed1 l\u01b0\u1ee3ng trung b\u00ecnh:<\/strong> Cardinality trung b\u00ecnh n\u1eb1m gi\u1eefa Cardinality th\u1ea5p v\u00e0 cao. C\u00e1c c\u1ed9t c\u00f3 Cardinality trung b\u00ecnh c\u00f3 s\u1ed1 l\u01b0\u1ee3ng gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t v\u1eeba ph\u1ea3i, khi\u1ebfn ch\u00fang c\u00f3 t\u00ednh ch\u1ecdn l\u1ecdc cao h\u01a1n c\u00e1c c\u1ed9t Cardinality th\u1ea5p nh\u01b0ng \u00edt ch\u1ecdn l\u1ecdc h\u01a1n c\u00e1c c\u1ed9t Cardinality cao.<\/p>\n<\/li>\n<\/ol>\n<p>D\u01b0\u1edbi \u0111\u00e2y l\u00e0 so s\u00e1nh ba lo\u1ea1i Cardinality:<\/p>\n<table>\n<thead>\n<tr>\n<th>Lo\u1ea1i s\u1ed1 l\u01b0\u1ee3ng<\/th>\n<th>S\u1ed1 l\u01b0\u1ee3ng gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t<\/th>\n<th>T\u00ednh ch\u1ecdn l\u1ecdc<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Th\u1ea5p<\/td>\n<td>M\u1ed9t v\u00e0i<\/td>\n<td>Th\u1ea5p<\/td>\n<\/tr>\n<tr>\n<td>Trung b\u00ecnh<\/td>\n<td>V\u1eeba ph\u1ea3i<\/td>\n<td>Trung b\u00ecnh<\/td>\n<\/tr>\n<tr>\n<td>Cao<\/td>\n<td>Nhi\u1ec1u<\/td>\n<td>Cao<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2>C\u00e1ch s\u1eed d\u1ee5ng Cardinality (SQL), 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>C\u00e1c c\u00e1ch s\u1eed d\u1ee5ng Cardinality trong SQL<\/h3>\n<ol>\n<li>\n<p><strong>T\u1ed1i \u01b0u h\u00f3a hi\u1ec7u su\u1ea5t truy v\u1ea5n:<\/strong> Cardinality gi\u00fap tr\u00ecnh t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n ch\u1ecdn ph\u01b0\u01a1ng \u00e1n th\u1ef1c hi\u1ec7n hi\u1ec7u qu\u1ea3 nh\u1ea5t, mang l\u1ea1i hi\u1ec7u su\u1ea5t truy v\u1ea5n nhanh h\u01a1n.<\/p>\n<\/li>\n<li>\n<p><strong>L\u1ef1a ch\u1ecdn ch\u1ec9 s\u1ed1:<\/strong> B\u1eb1ng c\u00e1ch ph\u00e2n t\u00edch Cardinality, b\u1ea1n c\u00f3 th\u1ec3 \u0111\u01b0a ra quy\u1ebft \u0111\u1ecbnh s\u00e1ng su\u1ed1t v\u1ec1 vi\u1ec7c l\u1eadp ch\u1ec9 m\u1ee5c c\u1ed9t n\u00e0o \u0111\u1ec3 c\u00f3 hi\u1ec7u su\u1ea5t truy v\u1ea5n t\u1ed1t h\u01a1n.<\/p>\n<\/li>\n<li>\n<p><strong>Ph\u00e2n t\u00edch ch\u1ea5t l\u01b0\u1ee3ng d\u1eef li\u1ec7u:<\/strong> Cardinality h\u1ed7 tr\u1ee3 x\u00e1c \u0111\u1ecbnh d\u1eef li\u1ec7u tr\u00f9ng l\u1eb7p ho\u1eb7c b\u1ecb thi\u1ebfu, \u0111i\u1ec1u n\u00e0y c\u00f3 th\u1ec3 r\u1ea5t quan tr\u1ecdng cho vi\u1ec7c l\u00e0m s\u1ea1ch v\u00e0 b\u1ea3o tr\u00ec d\u1eef li\u1ec7u.<\/p>\n<\/li>\n<\/ol>\n<h3>C\u00e1c v\u1ea5n \u0111\u1ec1 v\u00e0 gi\u1ea3i ph\u00e1p li\u00ean quan \u0111\u1ebfn Cardinality trong SQL<\/h3>\n<ol>\n<li>\n<p><strong>Th\u1ed1ng k\u00ea l\u1ed7i th\u1eddi:<\/strong> S\u1ed1 li\u1ec7u th\u1ed1ng k\u00ea l\u1ed7i th\u1eddi ho\u1eb7c kh\u00f4ng ch\u00ednh x\u00e1c c\u00f3 th\u1ec3 d\u1eabn \u0111\u1ebfn c\u00e1c k\u1ebf ho\u1ea1ch truy v\u1ea5n kh\u00f4ng t\u1ed1i \u01b0u. Th\u01b0\u1eddng xuy\u00ean c\u1eadp nh\u1eadt s\u1ed1 li\u1ec7u th\u1ed1ng k\u00ea c\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u1ec3 \u0111\u1ea3m b\u1ea3o \u01b0\u1edbc t\u00ednh Cardinality ch\u00ednh x\u00e1c.<\/p>\n<\/li>\n<li>\n<p><strong>Ph\u00e2n ph\u1ed1i d\u1eef li\u1ec7u sai l\u1ec7ch:<\/strong> Ph\u00e2n ph\u1ed1i d\u1eef li\u1ec7u sai l\u1ec7ch, trong \u0111\u00f3 m\u1ed9t gi\u00e1 tr\u1ecb chi\u1ebfm \u01b0u th\u1ebf trong m\u1ed9t c\u1ed9t, c\u00f3 th\u1ec3 d\u1eabn \u0111\u1ebfn c\u00e1c k\u1ebf ho\u1ea1ch truy v\u1ea5n kh\u00f4ng hi\u1ec7u qu\u1ea3. H\u00e3y xem x\u00e9t vi\u1ec7c ph\u00e2n v\u00f9ng ho\u1eb7c l\u1eadp ch\u1ec9 m\u1ee5c \u0111\u1ec3 x\u1eed l\u00fd c\u00e1c t\u00ecnh hu\u1ed1ng nh\u01b0 v\u1eady.<\/p>\n<\/li>\n<li>\n<p><strong>K\u00edch th\u01b0\u1edbc th\u00f9ng bi\u1ec3u \u0111\u1ed3:<\/strong> Bi\u1ec3u \u0111\u1ed3 \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 \u01b0\u1edbc t\u00ednh Cardinality c\u00f3 th\u1ec3 c\u00f3 k\u00edch th\u01b0\u1edbc th\u00f9ng kh\u00e1c nhau, d\u1eabn \u0111\u1ebfn \u01b0\u1edbc t\u00ednh Cardinality kh\u00f4ng ch\u00ednh x\u00e1c. Vi\u1ec7c \u0111i\u1ec1u ch\u1ec9nh k\u00edch th\u01b0\u1edbc th\u00f9ng bi\u1ec3u \u0111\u1ed3 c\u00f3 th\u1ec3 c\u1ea3i thi\u1ec7n \u0111\u1ed9 ch\u00ednh x\u00e1c.<\/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<\/h2>\n<h3>S\u1ed1 l\u01b0\u1ee3ng so v\u1edbi m\u1eadt \u0111\u1ed9<\/h3>\n<p>S\u1ed1 l\u01b0\u1ee3ng v\u00e0 M\u1eadt \u0111\u1ed9 l\u00e0 hai kh\u00e1i ni\u1ec7m thi\u1ebft y\u1ebfu \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng trong t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n, nh\u01b0ng ch\u00fang ph\u1ee5c v\u1ee5 c\u00e1c m\u1ee5c \u0111\u00edch kh\u00e1c nhau:<\/p>\n<ul>\n<li>\n<p><strong>H\u1ed3ng y<\/strong> \u0111\u1ec1 c\u1eadp \u0111\u1ebfn s\u1ed1 l\u01b0\u1ee3ng gi\u00e1 tr\u1ecb ri\u00eang bi\u1ec7t trong m\u1ed9t c\u1ed9t ho\u1eb7c m\u1ed9t ch\u1ec9 m\u1ee5c, h\u1ed7 tr\u1ee3 tr\u00ecnh t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n \u01b0\u1edbc t\u00ednh s\u1ed1 l\u01b0\u1ee3ng h\u00e0ng \u0111\u01b0\u1ee3c truy v\u1ea5n tr\u1ea3 v\u1ec1.<\/p>\n<\/li>\n<li>\n<p><strong>T\u1ec9 tr\u1ecdng<\/strong> th\u1ec3 hi\u1ec7n t\u00ednh duy nh\u1ea5t c\u1ee7a c\u00e1c gi\u00e1 tr\u1ecb d\u1eef li\u1ec7u trong m\u1ed9t ch\u1ec9 m\u1ee5c. N\u00f3 l\u00e0 ngh\u1ecbch \u0111\u1ea3o c\u1ee7a Cardinality, cho bi\u1ebft kh\u1ea3 n\u0103ng hai h\u00e0ng \u0111\u01b0\u1ee3c ch\u1ecdn ng\u1eabu nhi\u00ean c\u00f3 c\u00f9ng gi\u00e1 tr\u1ecb cho c\u1ed9t \u0111\u01b0\u1ee3c l\u1eadp ch\u1ec9 m\u1ee5c.<\/p>\n<\/li>\n<\/ul>\n<p>M\u1eb7c d\u00f9 c\u1ea3 Cardinality v\u00e0 Density \u0111\u1ec1u t\u00e1c \u0111\u1ed9ng \u0111\u1ebfn vi\u1ec7c t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n nh\u01b0ng ch\u00fang cung c\u1ea5p th\u00f4ng tin ri\u00eang bi\u1ec7t cho tr\u00ecnh t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n \u0111\u1ec3 l\u1ef1a ch\u1ecdn k\u1ebf ho\u1ea1ch truy v\u1ea5n hi\u1ec7u qu\u1ea3.<\/p>\n<h2>C\u00e1c quan \u0111i\u1ec3m v\u00e0 c\u00f4ng ngh\u1ec7 c\u1ee7a t\u01b0\u01a1ng lai li\u00ean quan \u0111\u1ebfn Cardinality (SQL)<\/h2>\n<p>Khi ti\u1ebfn b\u1ed9 c\u00f4ng ngh\u1ec7 v\u00e0 c\u01a1 s\u1edf d\u1eef li\u1ec7u tr\u1edf n\u00ean ph\u1ee9c t\u1ea1p h\u01a1n, t\u1ea7m quan tr\u1ecdng c\u1ee7a Cardinality trong SQL s\u1ebd ti\u1ebfp t\u1ee5c t\u0103ng l\u00ean. Nh\u1eefng ph\u00e1t tri\u1ec3n trong t\u01b0\u01a1ng lai v\u1ec1 thu\u1eadt to\u00e1n t\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n v\u00e0 c\u00e1c k\u1ef9 thu\u1eadt th\u1ed1ng k\u00ea n\u00e2ng cao \u0111\u01b0\u1ee3c k\u1ef3 v\u1ecdng s\u1ebd n\u00e2ng cao h\u01a1n n\u1eefa \u0111\u1ed9 ch\u00ednh x\u00e1c c\u1ee7a \u01b0\u1edbc t\u00ednh Cardinality. Ngo\u00e0i ra, nh\u1eefng ti\u1ebfn b\u1ed9 trong ki\u1ebfn tr\u00fac ph\u1ea7n c\u1ee9ng v\u00e0 c\u01a1 s\u1edf d\u1eef li\u1ec7u s\u1ebd d\u1eabn \u0111\u1ebfn vi\u1ec7c t\u00ednh to\u00e1n Cardinality th\u1eadm ch\u00ed c\u00f2n hi\u1ec7u qu\u1ea3 h\u01a1n, c\u1ea3i thi\u1ec7n hi\u1ec7u su\u1ea5t t\u1ed5ng th\u1ec3 c\u1ee7a h\u1ec7 th\u1ed1ng c\u01a1 s\u1edf d\u1eef li\u1ec7u.<\/p>\n<h2>C\u00e1ch s\u1eed d\u1ee5ng ho\u1eb7c li\u00ean k\u1ebft m\u00e1y ch\u1ee7 proxy v\u1edbi Cardinality (SQL)<\/h2>\n<p>C\u00e1c m\u00e1y ch\u1ee7 proxy, gi\u1ed1ng nh\u01b0 c\u00e1c m\u00e1y ch\u1ee7 do OneProxy cung c\u1ea5p, \u0111\u00f3ng vai tr\u00f2 quan tr\u1ecdng trong vi\u1ec7c n\u00e2ng cao quy\u1ec1n ri\u00eang t\u01b0, b\u1ea3o m\u1eadt v\u00e0 hi\u1ec7u su\u1ea5t khi truy c\u1eadp t\u00e0i nguy\u00ean web. M\u1eb7c d\u00f9 kh\u00f4ng li\u00ean quan tr\u1ef1c ti\u1ebfp \u0111\u1ebfn Cardinality trong SQL, nh\u01b0ng m\u00e1y ch\u1ee7 proxy c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng k\u1ebft h\u1ee3p v\u1edbi c\u00e1c \u1ee9ng d\u1ee5ng c\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u1ec3 c\u1ea3i thi\u1ec7n kh\u1ea3 n\u0103ng truy c\u1eadp v\u00e0 t\u00ednh kh\u1ea3 d\u1ee5ng c\u1ee7a d\u1eef li\u1ec7u.<\/p>\n<p>M\u00e1y ch\u1ee7 proxy c\u00f3 th\u1ec3 l\u01b0u tr\u1eef c\u00e1c t\u00e0i nguy\u00ean c\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u01b0\u1ee3c truy c\u1eadp th\u01b0\u1eddng xuy\u00ean v\u00e0o b\u1ed9 \u0111\u1ec7m, gi\u1ea3m s\u1ed1 l\u01b0\u1ee3ng y\u00eau c\u1ea7u \u0111\u1ebfn m\u00e1y ch\u1ee7 c\u01a1 s\u1edf d\u1eef li\u1ec7u v\u00e0 c\u00f3 kh\u1ea3 n\u0103ng c\u1ea3i thi\u1ec7n th\u1eddi gian ph\u1ea3n h\u1ed3i. Ngo\u00e0i ra, m\u00e1y ch\u1ee7 proxy c\u00f3 th\u1ec3 \u0111\u00f3ng vai tr\u00f2 trung gian gi\u1eefa m\u00e1y kh\u00e1ch v\u00e0 c\u01a1 s\u1edf d\u1eef li\u1ec7u, b\u1ed5 sung th\u00eam m\u1ed9t l\u1edbp b\u1ea3o m\u1eadt v\u00e0 c\u00e2n b\u1eb1ng t\u1ea3i, \u0111i\u1ec1u n\u00e0y c\u00f3 th\u1ec3 \u0111\u1eb7c bi\u1ec7t h\u1eefu \u00edch trong c\u00e1c t\u00ecnh hu\u1ed1ng c\u00f3 l\u01b0u l\u01b0\u1ee3ng truy c\u1eadp cao.<\/p>\n<h2>Li\u00ean k\u1ebft li\u00ean quan<\/h2>\n<p>\u0110\u1ec3 bi\u1ebft th\u00eam th\u00f4ng tin v\u1ec1 Cardinality trong SQL, b\u1ea1n c\u00f3 th\u1ec3 th\u1ea5y c\u00e1c t\u00e0i nguy\u00ean sau h\u1eefu \u00edch:<\/p>\n<ul>\n<li><a href=\"https:\/\/docs.microsoft.com\/en-us\/previous-versions\/sql\/sql-server-2008\/dd535534(v=sql.100)\" target=\"_new\" rel=\"noopener nofollow\">Hi\u1ec3u \u01b0\u1edbc t\u00ednh s\u1ed1 l\u01b0\u1ee3ng m\u00e1y ch\u1ee7 SQL<\/a><\/li>\n<li><a href=\"https:\/\/www.postgresql.org\/docs\/current\/planner-stats.html\" target=\"_new\" rel=\"noopener nofollow\">\u01af\u1edbc t\u00ednh s\u1ed1 l\u01b0\u1ee3ng trong PostgreSQL<\/a><\/li>\n<li><a href=\"https:\/\/dev.mysql.com\/doc\/mysql\/en\/query-optimization.html\" target=\"_new\" rel=\"noopener nofollow\">T\u1ed1i \u01b0u h\u00f3a truy v\u1ea5n MySQL v\u00e0 t\u00ednh ch\u00ednh x\u00e1c<\/a><\/li>\n<\/ul>\n<p>H\u00e3y nh\u1edb r\u1eb1ng, vi\u1ec7c hi\u1ec3u Cardinality l\u00e0 r\u1ea5t quan tr\u1ecdng \u0111\u1ec3 t\u1ed1i \u01b0u h\u00f3a hi\u1ec7u su\u1ea5t c\u01a1 s\u1edf d\u1eef li\u1ec7u v\u00e0 \u0111\u1ea3m b\u1ea3o th\u1ef1c thi truy v\u1ea5n hi\u1ec7u qu\u1ea3. Vi\u1ec7c theo k\u1ecbp nh\u1eefng ph\u00e1t tri\u1ec3n m\u1edbi nh\u1ea5t trong c\u00f4ng ngh\u1ec7 c\u01a1 s\u1edf d\u1eef li\u1ec7u s\u1ebd ti\u1ebfp th\u00eam s\u1ee9c m\u1ea1nh cho b\u1ea1n \u0111\u1ec3 \u0111\u01b0a ra nh\u1eefng quy\u1ebft \u0111\u1ecbnh s\u00e1ng su\u1ed1t v\u00e0 khai th\u00e1c to\u00e0n b\u1ed9 ti\u1ec1m n\u0103ng c\u1ee7a c\u00e1c \u1ee9ng d\u1ee5ng d\u1ef1a tr\u00ean d\u1eef li\u1ec7u c\u1ee7a b\u1ea1n.<\/p>","protected":false},"featured_media":467828,"menu_order":0,"template":"","meta":{"_acf_changed":false,"content-type":"","inline_featured_image":false,"footnotes":""},"class_list":["post-476178","wiki","type-wiki","status-publish","has-post-thumbnail","hentry"],"acf":{"faq_title":"Frequently Asked Questions about <mark>Cardinality (SQL)<\/mark>","faq_items":[{"question":"What is Cardinality in SQL?","answer":"<p>Cardinality in SQL refers to the number of distinct values present in a column or index of a database table. It helps the database engine optimize queries and make efficient execution plans.<\/p>"},{"question":"How does Cardinality work in SQL?","answer":"<p>Cardinality is maintained within the database statistics, which store information about the number of rows, distinct values, and data distribution. The query optimizer uses this information to estimate the number of rows returned by a query and choose the best execution plan.<\/p>"},{"question":"What are the types of Cardinality in SQL?","answer":"<p>There are three primary types of Cardinality:<\/p><ol><li>Low Cardinality: Few distinct values, often seen in columns like gender or country.<\/li><li>Medium Cardinality: Moderate distinct values, falling between low and high Cardinality.<\/li><li>High Cardinality: Many distinct values, common in primary key or unique identifier columns.<\/li><\/ol>"},{"question":"How can I use Cardinality in SQL?","answer":"<p>Cardinality is essential for:<\/p><ul><li>Optimizing query performance<\/li><li>Selecting appropriate indexes for better performance<\/li><li>Identifying data quality issues like duplicates or missing data<\/li><\/ul>"},{"question":"What are the challenges related to Cardinality in SQL?","answer":"<p>Problems related to Cardinality include outdated statistics, skewed data distribution, and inaccurate histogram bin sizes. Regularly updating statistics and considering partitioning or indexing can address these challenges.<\/p>"},{"question":"How is Cardinality different from Density in SQL?","answer":"<p>Cardinality represents the number of distinct values, while Density indicates the uniqueness of data values in an index. Both impact query optimization but serve different purposes.<\/p>"},{"question":"What is the future perspective of Cardinality in SQL?","answer":"<p>As technology advances, Cardinality's importance will continue to grow, leading to more accurate estimations and efficient query plans. Advancements in hardware and database architecture will further improve Cardinality computations and overall database performance.<\/p>"},{"question":"How can proxy servers be associated with Cardinality in SQL?","answer":"<p>While not directly related, proxy servers can work with database applications to improve data access and availability. They can cache frequently accessed resources, add security layers, and perform load balancing for high-traffic scenarios.<\/p>"}]},"_links":{"self":[{"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/wiki\/476178","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\/476178\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/media\/467828"}],"wp:attachment":[{"href":"https:\/\/oneproxy.pro\/vn\/wp-json\/wp\/v2\/media?parent=476178"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}