{"id":1120,"date":"2026-03-29T08:16:42","date_gmt":"2026-03-29T08:16:42","guid":{"rendered":"https:\/\/www.method-post.com\/vi\/decoding-multiplicity-class-diagrams-guide\/"},"modified":"2026-03-29T08:16:42","modified_gmt":"2026-03-29T08:16:42","slug":"decoding-multiplicity-class-diagrams-guide","status":"publish","type":"post","link":"https:\/\/www.method-post.com\/vi\/decoding-multiplicity-class-diagrams-guide\/","title":{"rendered":"Gi\u1ea3i m\u00e3 T\u00ednh \u0110a D\u1ea1ng: H\u01b0\u1edbng D\u1eabn \u0110\u01a1n Gi\u1ea3n \u0110\u1ec3 Th\u00e0nh Th\u1ea1o C\u00e1c M\u1ed1i Quan H\u1ec7 1:N, 1:1 v\u00e0 N:N"},"content":{"rendered":"<p>Trong b\u1ed1i c\u1ea3nh ki\u1ebfn tr\u00fac ph\u1ea7n m\u1ec1m v\u00e0 m\u00f4 h\u00ecnh h\u00f3a d\u1eef li\u1ec7u, \u00edt kh\u00e1i ni\u1ec7m n\u00e0o mang tr\u1ecdng l\u01b0\u1ee3ng b\u1eb1ng c\u00e1c m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c th\u1ef1c th\u1ec3. Khi thi\u1ebft k\u1ebf m\u1ed9t h\u1ec7 th\u1ed1ng, vi\u1ec7c hi\u1ec3u c\u00e1ch c\u00e1c \u0111\u1ed1i t\u01b0\u1ee3ng t\u01b0\u01a1ng t\u00e1c l\u00e0 quan tr\u1ecdng kh\u00f4ng k\u00e9m g\u00ec vi\u1ec7c \u0111\u1ecbnh ngh\u0129a ch\u00ednh b\u1ea3n th\u00e2n c\u00e1c \u0111\u1ed1i t\u01b0\u1ee3ng. S\u1ef1 t\u01b0\u01a1ng t\u00e1c n\u00e0y \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n m\u1ed9t c\u00e1ch ch\u00ednh th\u1ee9c th\u00f4ng qua<strong>t\u00ednh \u0111a d\u1ea1ng trong s\u01a1 \u0111\u1ed3 l\u1edbp<\/strong>, m\u1ed9t k\u00fd hi\u1ec7u quy \u0111\u1ecbnh m\u1ed1i quan h\u1ec7 \u0111\u1ecbnh l\u01b0\u1ee3ng gi\u1eefa hai l\u1edbp. D\u00f9 b\u1ea1n \u0111ang thi\u1ebft k\u1ebf s\u01a1 \u0111\u1ed3 c\u01a1 s\u1edf d\u1eef li\u1ec7u hay c\u1ea5u tr\u00fac m\u1ed9t m\u00e3 ngu\u1ed3n h\u01b0\u1edbng \u0111\u1ed1i t\u01b0\u1ee3ng, s\u1ef1 r\u00f5 r\u00e0ng \u1edf \u0111\u00e2y s\u1ebd ng\u0103n ng\u1eeba n\u1ee3 ki\u1ebfn tr\u00fac ngay t\u1eeb \u0111\u1ea7u.<\/p>\n<p>T\u00ednh \u0111a d\u1ea1ng \u0111\u1ecbnh ngh\u0129a c\u00e1c r\u00e0ng bu\u1ed9c v\u1ec1 s\u1ed1 l\u01b0\u1ee3ng th\u1ec3 hi\u1ec7n c\u1ee7a m\u1ed9t l\u1edbp c\u00f3 th\u1ec3 li\u00ean k\u1ebft v\u1edbi c\u00e1c th\u1ec3 hi\u1ec7n c\u1ee7a m\u1ed9t l\u1edbp kh\u00e1c. N\u00f3 tr\u1ea3 l\u1eddi nh\u1eefng c\u00e2u h\u1ecfi c\u1ed1t l\u00f5i: M\u1ed9t ng\u01b0\u1eddi d\u00f9ng c\u00f3 th\u1ec3 s\u1edf h\u1eefu nhi\u1ec1u h\u1ed3 s\u01a1 kh\u00f4ng? M\u1ed9t \u0111\u01a1n h\u00e0ng c\u00f3 th\u1ec3 thu\u1ed9c v\u1ec1 nhi\u1ec1u kh\u00e1ch h\u00e0ng kh\u00f4ng? Nh\u1eefng ph\u00e2n bi\u1ec7t n\u00e0y \u0111\u1ecbnh h\u00ecnh lu\u1ed3ng d\u1eef li\u1ec7u v\u00e0 t\u00ednh to\u00e0n v\u1eb9n c\u1ee7a \u1ee9ng d\u1ee5ng. H\u01b0\u1edbng d\u1eabn n\u00e0y kh\u00e1m ph\u00e1 c\u00e1c cardinalities c\u1ed1t l\u00f5i\u20141:1, 1:N v\u00e0 N:N\u2014cung c\u1ea5p c\u00e1i nh\u00ecn chi ti\u1ebft v\u1ec1 c\u00e1ch tri\u1ec3n khai, h\u1ec7 qu\u1ea3 v\u00e0 nh\u1eefng sai l\u1ea7m ph\u1ed5 bi\u1ebfn.<\/p>\n<div class=\"wp-block-image\">\n<figure class=\"aligncenter\"><img alt=\"A playful child's drawing style infographic explaining class diagram multiplicity: one-to-one (1:1) shown as a person with one passport, one-to-many (1:N) as a tree with many apples, and many-to-many (N:N) as students connected to courses via a junction table, with simple UML notation symbols (1, *, 0..1) in bright crayon colors on a white background, teaching software architecture relationships in an intuitive visual way\" decoding=\"async\" src=\"https:\/\/www.method-post.com\/wp-content\/uploads\/2026\/03\/multiplicity-relationships-infographic-childs-drawing.jpg\"\/><\/figure>\n<\/div>\n<h2>Hi\u1ec3u R\u1ec5 N\u1ec1n T\u1ea3ng: K\u00fd Hi\u1ec7u v\u00e0 Thu\u1eadt Ng\u1eef \ud83e\udde9<\/h2>\n<p>Tr\u01b0\u1edbc khi \u0111i s\u00e2u v\u00e0o c\u00e1c lo\u1ea1i m\u1ed1i quan h\u1ec7 c\u1ee5 th\u1ec3, \u0111i\u1ec1u c\u1ea7n thi\u1ebft l\u00e0 ph\u1ea3i thi\u1ebft l\u1eadp t\u1eeb v\u1ef1ng \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng trong Ng\u00f4n ng\u1eef M\u00f4 h\u00ecnh H\u00f3a \u0110\u01a1n Nh\u1ea5t (UML) v\u00e0 m\u00f4 h\u00ecnh h\u00f3a d\u1eef li\u1ec7u n\u00f3i chung. T\u00ednh \u0111a d\u1ea1ng kh\u00f4ng ch\u1ec9 \u0111\u01a1n thu\u1ea7n l\u00e0 \u0111\u1ebfm; n\u00f3 l\u00e0 vi\u1ec7c \u0111\u1ecbnh ngh\u0129a c\u00e1c quy t\u1eafc.<\/p>\n<ul>\n<li><strong>Cardinality:<\/strong> S\u1ed1 l\u01b0\u1ee3ng th\u1ec3 hi\u1ec7n c\u1ee7a m\u1ed9t l\u1edbp c\u00f3 th\u1ec3 tham gia v\u00e0o m\u1ed9t m\u1ed1i quan h\u1ec7. Th\u01b0\u1eddng \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n b\u1eb1ng c\u00e1c con s\u1ed1 nh\u01b0<code>1<\/code>, <code>*<\/code>, ho\u1eb7c c\u00e1c kho\u1ea3ng nh\u01b0<code>0..1<\/code>.<\/li>\n<li><strong>T\u00ednh t\u00f9y ch\u1ecdn:<\/strong> Li\u1ec7u m\u1ed9t th\u1ec3 hi\u1ec7n c\u1ee7a m\u1ed9t l\u1edbp c\u00f3 b\u1eaft bu\u1ed9c ph\u1ea3i tham gia v\u00e0o m\u1ed1i quan h\u1ec7 hay kh\u00f4ng. V\u00ed d\u1ee5: M\u1ed7i nh\u00e2n vi\u00ean c\u00f3 c\u1ea7n m\u1ed9t ng\u01b0\u1eddi qu\u1ea3n l\u00fd kh\u00f4ng?<\/li>\n<li><strong>Li\u00ean k\u1ebft:<\/strong> Ch\u00ednh li\u00ean k\u1ebft \u0111\u00f3, \u0111\u1ea1i di\u1ec7n cho m\u1ed1i quan h\u1ec7 c\u1ea5u tr\u00fac gi\u1eefa c\u00e1c l\u1edbp.<\/li>\n<\/ul>\n<p>Khi b\u1ea1n nh\u00ecn v\u00e0o m\u1ed9t s\u01a1 \u0111\u1ed3 l\u1edbp, b\u1ea1n s\u1ebd th\u1ea5y c\u00e1c \u0111\u01b0\u1eddng n\u1ed1i gi\u1eefa c\u00e1c h\u1ed9p. G\u1ea7n nh\u1eefng \u0111\u01b0\u1eddng n\u00e0y, c\u00e1c con s\u1ed1 nh\u1ecf ho\u1eb7c k\u00fd hi\u1ec7u cho th\u1ea5y t\u00ednh \u0111a d\u1ea1ng. Nh\u1eefng k\u00fd hi\u1ec7u n\u00e0y ho\u1ea1t \u0111\u1ed9ng nh\u01b0 c\u00e1c h\u1ee3p \u0111\u1ed3ng. N\u1ebfu logic h\u1ec7 th\u1ed1ng vi ph\u1ea1m nh\u1eefng h\u1ee3p \u0111\u1ed3ng n\u00e0y, d\u1eef li\u1ec7u s\u1ebd tr\u1edf n\u00ean kh\u00f4ng nh\u1ea5t qu\u00e1n. Hi\u1ec3u \u0111\u01b0\u1ee3c k\u00fd hi\u1ec7u n\u00e0y l\u00e0 b\u01b0\u1edbc \u0111\u1ea7u ti\u00ean h\u01b0\u1edbng t\u1edbi thi\u1ebft k\u1ebf v\u1eefng ch\u1eafc.<\/p>\n<h2>M\u1ed1i quan h\u1ec7 M\u1ed9t-\u0110\u1ed1i-M\u1ed9t (1:1) \ud83d\udd17<\/h2>\n<p>M\u1ed1i quan h\u1ec7 m\u1ed9t-\u0111\u1ed1i-m\u1ed9t l\u00e0 lo\u1ea1i li\u00ean k\u1ebft ti\u00eau chu\u1ea9n kh\u1eaft khe nh\u1ea5t. N\u00f3 ng\u1ee5 \u00fd r\u1eb1ng v\u1edbi m\u1ed7i th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp A, s\u1ebd c\u00f3 nhi\u1ec1u nh\u1ea5t m\u1ed9t th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp B, v\u00e0 ng\u01b0\u1ee3c l\u1ea1i. \u0110i\u1ec1u n\u00e0y th\u01b0\u1eddng \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n b\u1eb1ng k\u00fd hi\u1ec7u<code>1<\/code> \u1edf c\u1ea3 hai \u0111\u1ea7u c\u1ee7a \u0111\u01b0\u1eddng li\u00ean k\u1ebft.<\/p>\n<h3>Khi n\u00e0o n\u00ean s\u1eed d\u1ee5ng c\u00e1c li\u00ean k\u1ebft 1:1<\/h3>\n<p>Lo\u1ea1i m\u1ed1i quan h\u1ec7 n\u00e0y ph\u00f9 h\u1ee3p khi hai kh\u00e1i ni\u1ec7m th\u1ef1c ch\u1ea5t l\u00e0 nh\u1eefng g\u00f3c nh\u00ecn kh\u00e1c nhau v\u1ec1 c\u00f9ng m\u1ed9t th\u1ef1c th\u1ec3, ho\u1eb7c khi li\u00ean k\u1ebft l\u00e0 \u0111\u1ed9c quy\u1ec1n v\u00e0 v\u0129nh vi\u1ec5n.<\/p>\n<ul>\n<li><strong>Ch\u00eca Kh\u00f3a X\u00e1c Th\u1ef1c:<\/strong> M\u1ed9t t\u00e0i kho\u1ea3n Ng\u01b0\u1eddi d\u00f9ng c\u00f3 th\u1ec3 c\u00f3 \u0111\u00fang m\u1ed9t m\u00e3 th\u00f4ng b\u00e1o phi\u00ean ho\u1ea1t \u0111\u1ed9ng t\u1ea1i m\u1ed9t th\u1eddi \u0111i\u1ec3m. N\u1ebfu ng\u01b0\u1eddi d\u00f9ng \u0111\u0103ng nh\u1eadp l\u1ea1i, m\u00e3 th\u00f4ng b\u00e1o tr\u01b0\u1edbc \u0111\u00f3 s\u1ebd b\u1ecb v\u00f4 hi\u1ec7u h\u00f3a.<\/li>\n<li><strong>T\u00e0i li\u1ec7u Nh\u1eadn d\u1ea1ng:<\/strong> M\u1ed9t H\u1ed9 chi\u1ebfu \u0111\u01b0\u1ee3c c\u1ea5p cho m\u1ed9t C\u00f4ng d\u00e2n c\u1ee5 th\u1ec3, v\u00e0 m\u1ed9t C\u00f4ng d\u00e2n ch\u1ec9 gi\u1eef m\u1ed9t H\u1ed9 chi\u1ebfu ch\u00ednh th\u1ee9c t\u1ea1i m\u1ed9t th\u1eddi \u0111i\u1ec3m.<\/li>\n<li><strong>C\u00e0i \u0111\u1eb7t C\u1ea5u h\u00ecnh:<\/strong>M\u1ed9t phi\u00ean b\u1ea3n \u1ee8ng d\u1ee5ng c\u1ee5 th\u1ec3 th\u01b0\u1eddng c\u00f3 m\u1ed9t \u0111\u1ed1i t\u01b0\u1ee3ng C\u1ea5u h\u00ecnh duy nh\u1ea5t l\u01b0u tr\u1eef c\u00e1c tham s\u1ed1 th\u1eddi gian ch\u1ea1y c\u1ee7a n\u00f3.<\/li>\n<\/ul>\n<h3>Xem x\u00e9t v\u1ec1 Tri\u1ec3n khai<\/h3>\n<p>Tri\u1ec3n khai m\u1ed1i quan h\u1ec7 1:1 \u0111\u00f2i h\u1ecfi s\u1ef1 ch\u00fa \u00fd c\u1ea9n th\u1eadn \u0111\u1ebfn c\u00e1c kh\u00f3a ngo\u1ea1i v\u00e0 r\u00e0ng bu\u1ed9c c\u01a1 s\u1edf d\u1eef li\u1ec7u. Trong ng\u1eef c\u1ea3nh c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7, \u0111i\u1ec1u n\u00e0y th\u01b0\u1eddng \u0111\u01b0\u1ee3c th\u1ef1c hi\u1ec7n b\u1eb1ng c\u00e1ch \u0111\u1eb7t m\u1ed9t kh\u00f3a ngo\u1ea1i trong m\u1ed9t trong c\u00e1c b\u1ea3ng tham chi\u1ebfu \u0111\u1ebfn kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng kia.<\/p>\n<ul>\n<li><strong>Kh\u00f3a ngo\u1ea1i C\u01a1 s\u1edf d\u1eef li\u1ec7u:<\/strong>B\u1ea1n ph\u1ea3i th\u00eam m\u1ed9t <code>KH\u00d3A NGO\u1ea0I<\/code>r\u00e0ng bu\u1ed9c \u0111\u1ec3 \u0111\u1ea3m b\u1ea3o t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu. \u0110i\u1ec1u n\u00e0y ng\u0103n ch\u1eb7n c\u00e1c b\u1ea3n ghi b\u1ecb t\u00e1ch r\u1eddi.<\/li>\n<li><strong>R\u00e0ng bu\u1ed9c Duy nh\u1ea5t:<\/strong>\u0110\u1ec3 \u0111\u1ea3m b\u1ea3o nghi\u00eam ng\u1eb7t ph\u00eda &#8220;m\u1ed9t&#8221;, c\u1ed9t ch\u1ee9a kh\u00f3a ngo\u1ea1i ph\u1ea3i c\u00f3 m\u1ed9t <code>DUY NH\u1ea4T<\/code>r\u00e0ng bu\u1ed9c. \u0110i\u1ec1u n\u00e0y \u0111\u1ea3m b\u1ea3o kh\u00f4ng c\u00f3 hai h\u00e0ng n\u00e0o c\u00f3 th\u1ec3 tr\u1ecf \u0111\u1ebfn c\u00f9ng m\u1ed9t cha m\u1eb9.<\/li>\n<li><strong>Tham chi\u1ebfu M\u00e3 ngu\u1ed3n:<\/strong>Trong m\u00e3 ngu\u1ed3n h\u01b0\u1edbng \u0111\u1ed1i t\u01b0\u1ee3ng, \u0111i\u1ec1u n\u00e0y th\u01b0\u1eddng th\u1ec3 hi\u1ec7n d\u01b0\u1edbi d\u1ea1ng tham chi\u1ebfu tr\u1ef1c ti\u1ebfp \u0111\u1ebfn m\u1ed9t \u0111\u1ed1i t\u01b0\u1ee3ng duy nh\u1ea5t thay v\u00ec m\u1ed9t t\u1eadp h\u1ee3p. L\u1edbp <code>User<\/code> c\u00f3 th\u1ec3 c\u00f3 m\u1ed9t thu\u1ed9c t\u00ednh <code>Profile<\/code> ki\u1ec3u <code>Profile<\/code>, kh\u00f4ng ph\u1ea3i <code>List&lt;Profile&gt;<\/code>.<\/li>\n<\/ul>\n<h2>M\u1ed1i quan h\u1ec7 M\u1ed9t-\u0110a (1:N) \ud83c\udf33<\/h2>\n<p>M\u1ed1i quan h\u1ec7 m\u1ed9t-\u0111a l\u00e0 m\u1ed1i quan h\u1ec7 ph\u1ed5 bi\u1ebfn nh\u1ea5t trong c\u00e1c h\u1ec7 th\u1ed1ng doanh nghi\u1ec7p. \u1ede \u0111\u00e2y, m\u1ed9t th\u1ec3 hi\u1ec7n duy nh\u1ea5t c\u1ee7a L\u1edbp A \u0111\u01b0\u1ee3c li\u00ean k\u1ebft v\u1edbi kh\u00f4ng ho\u1eb7c nhi\u1ec1u th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp B. Tuy nhi\u00ean, m\u1ed7i th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp B \u0111\u01b0\u1ee3c li\u00ean k\u1ebft v\u1edbi \u0111\u00fang m\u1ed9t th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp A. K\u00fd hi\u1ec7u th\u01b0\u1eddng hi\u1ec3n th\u1ecb <code>1<\/code> \u1edf m\u1ed9t \u0111\u1ea7u v\u00e0 <code>*<\/code> (ho\u1eb7c <code>0..*<\/code>) \u1edf \u0111\u1ea7u kia.<\/p>\n<h3>C\u00e1c t\u00ecnh hu\u1ed1ng ph\u1ed5 bi\u1ebfn<\/h3>\n<p>M\u1eabu n\u00e0y m\u00f4 t\u1ea3 d\u1eef li\u1ec7u ph\u00e2n c\u1ea5p n\u01a1i m\u1ed9t n\u00fat cha s\u1edf h\u1eefu nhi\u1ec1u n\u00fat con.<\/p>\n<ul>\n<li><strong>\u0110\u01a1n h\u00e0ng v\u00e0 c\u00e1c m\u1ee5c h\u00e0ng:<\/strong> M\u1ed9t \u0111\u01a1n h\u00e0ng duy nh\u1ea5t ch\u1ee9a nhi\u1ec1u m\u1ee5c h\u00e0ng, nh\u01b0ng m\u1ed7i m\u1ee5c h\u00e0ng ch\u1ec9 thu\u1ed9c v\u1ec1 m\u1ed9t \u0111\u01a1n h\u00e0ng duy nh\u1ea5t.<\/li>\n<li><strong>Ph\u00f2ng ban v\u00e0 nh\u00e2n vi\u00ean:<\/strong> M\u1ed9t ph\u00f2ng ban tuy\u1ec3n d\u1ee5ng nhi\u1ec1u nh\u00e2n vi\u00ean, nh\u01b0ng m\u1ed7i nh\u00e2n vi\u00ean ch\u1ec9 \u0111\u01b0\u1ee3c g\u00e1n v\u00e0o m\u1ed9t ph\u00f2ng ban (trong c\u1ea5u tr\u00fac \u0111\u01a1n gi\u1ea3n).<\/li>\n<li><strong>Danh m\u1ee5c v\u00e0 s\u1ea3n ph\u1ea9m:<\/strong> M\u1ed9t danh m\u1ee5c s\u1ea3n ph\u1ea9m bao g\u1ed3m nhi\u1ec1u s\u1ea3n ph\u1ea9m, nh\u01b0ng m\u1ed7i s\u1ea3n ph\u1ea9m thu\u1ed9c v\u1ec1 m\u1ed9t danh m\u1ee5c c\u1ee5 th\u1ec3.<\/li>\n<\/ul>\n<h3>C\u1ea5u tr\u00fac d\u1eef li\u1ec7u<\/h3>\n<p>Th\u1ef1c hi\u1ec7n m\u1ed1i quan h\u1ec7 1:N kh\u00e1 \u0111\u01a1n gi\u1ea3n trong c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7 nh\u01b0ng \u0111\u00f2i h\u1ecfi x\u1eed l\u00fd \u0111\u1eb7c bi\u1ec7t trong c\u00e1c m\u00f4 h\u00ecnh b\u1ed9 nh\u1edb.<\/p>\n<ul>\n<li><strong>V\u1ecb tr\u00ed \u0111\u1eb7t kh\u00f3a ngo\u1ea1i:<\/strong> Kh\u00f3a ngo\u1ea1i n\u1eb1m \u1edf ph\u00eda &#8220;nhi\u1ec1u&#8221; (b\u1ea3ng con). B\u1ea3ng \u0110\u01a1n h\u00e0ng s\u1ebd c\u00f3 m\u1ed9t c\u1ed9t <code>order_id<\/code> li\u00ean k\u1ebft \u0111\u1ebfn b\u1ea3ng C\u00e1c m\u1ee5c h\u00e0ng.<\/li>\n<li><strong>Qu\u1ea3n l\u00fd t\u1eadp h\u1ee3p:<\/strong> \u1ede ph\u00eda &#8220;m\u1ed9t&#8221; (\u0111\u1ed1i t\u01b0\u1ee3ng cha), b\u1ea1n th\u01b0\u1eddng duy tr\u00ec m\u1ed9t t\u1eadp h\u1ee3p. M\u1ed9t \u0111\u1ed1i t\u01b0\u1ee3ng <code>Kh\u00e1ch h\u00e0ng<\/code> s\u1ebd ch\u1ee9a m\u1ed9t danh s\u00e1ch ho\u1eb7c m\u1ea3ng c\u00e1c \u0111\u1ed1i t\u01b0\u1ee3ng <code>\u0110\u01a1n h\u00e0ng<\/code> \u0111\u1ed1i t\u01b0\u1ee3ng.<\/li>\n<li><strong>H\u1ec7 qu\u1ea3 v\u1ec1 hi\u1ec7u su\u1ea5t:<\/strong> Vi\u1ec7c truy xu\u1ea5t ph\u00eda &#8220;nhi\u1ec1u&#8221; c\u00f3 th\u1ec3 tr\u1edf n\u00ean t\u1ed1n k\u00e9m n\u1ebfu t\u1eadp h\u1ee3p l\u1edbn. Th\u01b0\u1eddng xuy\u00ean s\u1eed d\u1ee5ng t\u1ea3i ch\u1eadm (lazy loading) \u0111\u1ec3 ch\u1ec9 l\u1ea5y c\u00e1c \u0111\u1ed1i t\u01b0\u1ee3ng con khi \u0111\u01b0\u1ee3c truy c\u1eadp, gi\u1ea3m thi\u1ec3u chi ph\u00ed truy v\u1ea5n ban \u0111\u1ea7u.<\/li>\n<\/ul>\n<h3>X\u1eed l\u00fd x\u00f3a lan truy\u1ec1n<\/h3>\n<p>M\u1ed9t quy\u1ebft \u0111\u1ecbnh quan tr\u1ecdng trong thi\u1ebft k\u1ebf 1:N l\u00e0 \u0111i\u1ec1u g\u00ec x\u1ea3y ra khi n\u00fat cha b\u1ecb x\u00f3a. N\u1ebfu b\u1ea1n x\u00f3a m\u1ed9t ph\u00f2ng ban, b\u1ea1n c\u00f3 x\u00f3a t\u1ea5t c\u1ea3 nh\u00e2n vi\u00ean kh\u00f4ng? Th\u01b0\u1eddng th\u00ec c\u00e2u tr\u1ea3 l\u1eddi l\u00e0 kh\u00f4ng, nh\u01b0ng h\u1ec7 th\u1ed1ng ph\u1ea3i x\u1eed l\u00fd \u0111i\u1ec1u n\u00e0y.<\/p>\n<ul>\n<li><strong>X\u00f3a lan truy\u1ec1n:<\/strong> T\u1ef1 \u0111\u1ed9ng x\u00f3a t\u1ea5t c\u1ea3 b\u1ea3n ghi con khi n\u00fat cha b\u1ecb x\u00f3a. H\u1eefu \u00edch cho d\u1eef li\u1ec7u t\u1ea1m th\u1eddi nh\u01b0 Nh\u1eadt k\u00fd \u0110\u01a1n h\u00e0ng.<\/li>\n<li><strong>C\u1ea5m x\u00f3a:<\/strong> Ng\u0103n vi\u1ec7c x\u00f3a n\u00fat cha n\u1ebfu t\u1ed3n t\u1ea1i c\u00e1c n\u00fat con. H\u1eefu \u00edch cho d\u1eef li\u1ec7u c\u1ed1t l\u00f5i nh\u01b0 S\u1ea3n ph\u1ea9m.<\/li>\n<li><strong>\u0110\u1eb7t v\u1ec1 null:<\/strong> \u0110\u1eb7t kh\u00f3a ngo\u1ea1i trong n\u00fat con v\u1ec1 gi\u00e1 tr\u1ecb null. Y\u00eau c\u1ea7u n\u00fat con ph\u1ea3i cho ph\u00e9p gi\u00e1 tr\u1ecb null.<\/li>\n<\/ul>\n<h2>M\u1ed1i quan h\u1ec7 Nhi\u1ec1u-\u0111\u1ebfn-Nhi\u1ec1u (N:N) \ud83d\udd78\ufe0f<\/h2>\n<p>M\u1ed1i quan h\u1ec7 nhi\u1ec1u-\u0111\u1ebfn-nhi\u1ec1u l\u00e0 m\u1ed1i quan h\u1ec7 ph\u1ee9c t\u1ea1p nh\u1ea5t trong ba m\u1ed1i quan h\u1ec7. N\u00f3 x\u1ea3y ra khi c\u00e1c th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp A c\u00f3 th\u1ec3 li\u00ean k\u1ebft v\u1edbi nhi\u1ec1u th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp B, v\u00e0 c\u00e1c th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp B c\u0169ng c\u00f3 th\u1ec3 li\u00ean k\u1ebft v\u1edbi nhi\u1ec1u th\u1ec3 hi\u1ec7n c\u1ee7a L\u1edbp A. K\u00fd hi\u1ec7u th\u1ec3 hi\u1ec7n<code>*<\/code> (ho\u1eb7c <code>0..*<\/code>) \u1edf c\u1ea3 hai \u0111\u1ea7u.<\/p>\n<h3>V\u00ed d\u1ee5 th\u1ef1c t\u1ebf<\/h3>\n<p>M\u1ed1i quan h\u1ec7 n\u00e0y ph\u1ed5 bi\u1ebfn trong c\u00e1c t\u00ecnh hu\u1ed1ng li\u00ean quan \u0111\u1ebfn th\u1ebb, vai tr\u00f2 ho\u1eb7c \u0111\u0103ng k\u00fd.<\/p>\n<ul>\n<li><strong>Sinh vi\u00ean v\u00e0 Kh\u00f3a h\u1ecdc:<\/strong> M\u1ed9t Sinh vi\u00ean \u0111\u0103ng k\u00fd nhi\u1ec1u Kh\u00f3a h\u1ecdc, v\u00e0 m\u1ed9t Kh\u00f3a h\u1ecdc c\u00f3 nhi\u1ec1u Sinh vi\u00ean.<\/li>\n<li><strong>T\u00e1c gi\u1ea3 v\u00e0 S\u00e1ch:<\/strong> M\u1ed9t T\u00e1c gi\u1ea3 vi\u1ebft nhi\u1ec1u S\u00e1ch, v\u00e0 m\u1ed9t S\u00e1ch c\u00f3 th\u1ec3 c\u00f3 nhi\u1ec1u T\u00e1c gi\u1ea3 (t\u00e1c gi\u1ea3 \u0111\u1ed3ng th\u1eddi).<\/li>\n<li><strong>K\u1ef9 n\u0103ng v\u00e0 Nh\u00e2n vi\u00ean:<\/strong> M\u1ed9t Nh\u00e2n vi\u00ean c\u00f3 nhi\u1ec1u K\u1ef9 n\u0103ng, v\u00e0 m\u1ed9t K\u1ef9 n\u0103ng \u0111\u01b0\u1ee3c s\u1edf h\u1eefu b\u1edfi nhi\u1ec1u Nh\u00e2n vi\u00ean.<\/li>\n<\/ul>\n<h3>Gi\u1ea3i ph\u00e1p th\u1ef1c th\u1ec3 n\u1ed1i<\/h3>\n<p>Vi\u1ec7c tri\u1ec3n khai tr\u1ef1c ti\u1ebfp m\u1ed1i quan h\u1ec7 N:N trong c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7 l\u00e0 kh\u00f4ng th\u1ec3. M\u1ed9t kh\u00f3a ngo\u1ea1i \u0111\u01a1n l\u1ebb kh\u00f4ng th\u1ec3 li\u00ean k\u1ebft hai b\u1ea3ng theo c\u1ea3 hai chi\u1ec1u m\u00e0 kh\u00f4ng g\u00e2y nh\u1ea7m l\u1eabn. Gi\u1ea3i ph\u00e1p l\u00e0 gi\u1edbi thi\u1ec7u m\u1ed9t<strong>b\u1ea3ng n\u1ed1i<\/strong> (ho\u1eb7c th\u1ef1c th\u1ec3 li\u00ean k\u1ebft).<\/p>\n<p>B\u1ea3ng trung gian n\u00e0y chia m\u1ed1i quan h\u1ec7 N:N th\u00e0nh hai m\u1ed1i quan h\u1ec7 1:N.<\/p>\n<ul>\n<li><strong>C\u1ea5u tr\u00fac:<\/strong> B\u1ea3ng n\u1ed1i ch\u1ee9a c\u00e1c kh\u00f3a ch\u00ednh c\u1ee7a c\u1ea3 hai b\u1ea3ng li\u00ean quan d\u01b0\u1edbi d\u1ea1ng kh\u00f3a ngo\u1ea1i.<\/li>\n<li><strong>D\u1eef li\u1ec7u b\u1ed5 sung:<\/strong>Kh\u00e1c v\u1edbi m\u1ed9t li\u00ean k\u1ebft \u0111\u01a1n gi\u1ea3n, b\u1ea3ng n\u1ed1i c\u00f3 th\u1ec3 l\u01b0u tr\u1eef c\u00e1c thu\u1ed9c t\u00ednh ri\u00eang c\u1ee7a n\u00f3. V\u00ed d\u1ee5, li\u00ean k\u1ebft gi\u1eefa Sinh vi\u00ean v\u00e0 Kh\u00f3a h\u1ecdc c\u00f3 th\u1ec3 c\u1ea7n m\u1ed9t<code>\u0111i\u1ec3m s\u1ed1<\/code> ho\u1eb7c <code>ng\u00e0y \u0111\u0103ng k\u00fd<\/code>.<\/li>\n<li><strong>Kh\u00f3a h\u1ee3p th\u00e0nh:<\/strong> Kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng n\u1ed1i th\u01b0\u1eddng l\u00e0 m\u1ed9t kh\u00f3a h\u1ee3p th\u00e0nh g\u1ed3m hai kh\u00f3a ngo\u1ea1i, \u0111\u1ea3m b\u1ea3o s\u1ef1 gh\u00e9p \u0111\u00f4i duy nh\u1ea5t.<\/li>\n<\/ul>\n<h3>Tri\u1ec3n khai h\u01b0\u1edbng \u0111\u1ed1i t\u01b0\u1ee3ng<\/h3>\n<p>Trong m\u00e3 ngu\u1ed3n, vi\u1ec7c qu\u1ea3n l\u00fd c\u00e1c m\u1ed1i quan h\u1ec7 N:N \u0111\u00f2i h\u1ecfi ph\u1ea3i duy tr\u00ec t\u00ednh nh\u1ea5t qu\u00e1n hai chi\u1ec1u. N\u1ebfu b\u1ea1n th\u00eam m\u1ed9t kh\u00f3a h\u1ecdc v\u00e0o danh s\u00e1ch c\u1ee7a m\u1ed9t sinh vi\u00ean, b\u1ea1n c\u0169ng ph\u1ea3i th\u00eam sinh vi\u00ean \u0111\u00f3 v\u00e0o danh s\u00e1ch c\u1ee7a kh\u00f3a h\u1ecdc.<\/p>\n<ul>\n<li><strong>\u0110\u1ed3ng b\u1ed9 h\u00f3a:<\/strong>C\u00e1c ph\u01b0\u01a1ng th\u1ee9c h\u1ed7 tr\u1ee3 n\u00ean \u0111\u01b0\u1ee3c t\u1ea1o ra \u0111\u1ec3 qu\u1ea3n l\u00fd c\u00e1c li\u00ean k\u1ebft n\u00e0y. M\u1ed9t <code>Student.addCourse(Kh\u00f3a h\u1ecdc c)<\/code>ph\u01b0\u01a1ng th\u1ee9c n\u00e0y n\u00ean t\u1ef1 \u0111\u1ed9ng th\u00eam sinh vi\u00ean v\u00e0o danh s\u00e1ch c\u1ee7a kh\u00f3a h\u1ecdc.<\/li>\n<li><strong>S\u1eed d\u1ee5ng b\u1ed9 nh\u1edb:<\/strong>V\u00ec d\u1eef li\u1ec7u b\u1ecb tr\u00f9ng l\u1eb7p trong hai b\u1ed9 s\u01b0u t\u1eadp (danh s\u00e1ch sinh vi\u00ean v\u00e0 danh s\u00e1ch kh\u00f3a h\u1ecdc), vi\u1ec7c s\u1eed d\u1ee5ng b\u1ed9 nh\u1edb s\u1ebd t\u0103ng l\u00ean. \u0110\u1ea3m b\u1ea3o thu gom r\u00e1c x\u1eed l\u00fd c\u00e1c tham chi\u1ebfu b\u1ecb b\u1ecf r\u01a1i n\u1ebfu m\u1ed9t li\u00ean k\u1ebft b\u1ecb x\u00f3a.<\/li>\n<\/ul>\n<h2>S\u1ed1 l\u01b0\u1ee3ng (Cardinality) so v\u1edbi T\u00ednh t\u00f9y ch\u1ecdn (Optionality): M\u1ed9t s\u1ef1 ph\u00e2n bi\u1ec7t quan tr\u1ecdng \u2696\ufe0f<\/h2>\n<p>Trong khi th\u1ea3o lu\u1eadn v\u1ec1 t\u00ednh \u0111a d\u1ea1ng, \u0111i\u1ec1u r\u1ea5t quan tr\u1ecdng l\u00e0 ph\u1ea3i ph\u00e2n bi\u1ec7t gi\u1eefa s\u1ed1 l\u01b0\u1ee3ng bao nhi\u00eau v\u00e0 vi\u1ec7c c\u00f3 b\u1eaft bu\u1ed9c hay kh\u00f4ng. Hai kh\u00e1i ni\u1ec7m n\u00e0y th\u01b0\u1eddng b\u1ecb nh\u1ea7m l\u1eabn nh\u01b0ng \u0111\u1ea1i di\u1ec7n cho nh\u1eefng quy t\u1eafc kh\u00e1c nhau.<\/p>\n<ul>\n<li><strong>S\u1ed1 l\u01b0\u1ee3ng t\u1ed1i thi\u1ec3u:<\/strong> S\u1ed1 l\u01b0\u1ee3ng t\u1ed1i thi\u1ec3u c\u00e1c th\u1ec3 hi\u1ec7n \u0111\u01b0\u1ee3c y\u00eau c\u1ea7u. Th\u01b0\u1eddng l\u00e0 0 ho\u1eb7c 1.<\/li>\n<li><strong>S\u1ed1 l\u01b0\u1ee3ng t\u1ed1i \u0111a:<\/strong> S\u1ed1 l\u01b0\u1ee3ng t\u1ed1i \u0111a c\u00e1c th\u1ec3 hi\u1ec7n \u0111\u01b0\u1ee3c ph\u00e9p. Th\u01b0\u1eddng l\u00e0 1 ho\u1eb7c nhi\u1ec1u (*).<\/li>\n<li><strong>Kh\u00f4ng ho\u1eb7c M\u1ed9t (0..1):<\/strong>M\u1ed1i quan h\u1ec7 l\u00e0 t\u00f9y ch\u1ecdn. Th\u1ec3 hi\u1ec7n c\u00f3 th\u1ec3 t\u1ed3n t\u1ea1i ho\u1eb7c kh\u00f4ng t\u1ed3n t\u1ea1i.<\/li>\n<li><strong>M\u1ed9t ho\u1eb7c Nhi\u1ec1u h\u01a1n (1..*):<\/strong>M\u1ed1i quan h\u1ec7 l\u00e0 b\u1eaft bu\u1ed9c. Th\u1ec3 hi\u1ec7n ph\u1ea3i t\u1ed3n t\u1ea1i v\u00e0 c\u00f3 th\u1ec3 c\u00f3 nhi\u1ec1u.<\/li>\n<\/ul>\n<p>H\u00e3y xem x\u00e9t m\u1ed1i quan h\u1ec7 gi\u1eefa <code>Nh\u00e2n vi\u00ean<\/code> v\u00e0 <code>Qu\u1ea3n l\u00fd<\/code>m\u1ed1i quan h\u1ec7. M\u1ed9t nh\u00e2n vi\u00ean ph\u1ea3i c\u00f3 m\u1ed9t ng\u01b0\u1eddi qu\u1ea3n l\u00fd (1..1), nh\u01b0ng m\u1ed9t ng\u01b0\u1eddi qu\u1ea3n l\u00fd c\u00f3 th\u1ec3 kh\u00f4ng qu\u1ea3n l\u00fd ai v\u00e0o m\u1ed9t th\u1eddi \u0111i\u1ec3m c\u1ee5 th\u1ec3 (0..*). Hi\u1ec3u r\u00f5 nh\u1eefng s\u1eafc th\u00e1i n\u00e0y cho ph\u00e9p thi\u1ebft l\u1eadp c\u00e1c r\u00e0ng bu\u1ed9c c\u01a1 s\u1edf d\u1eef li\u1ec7u ch\u00ednh x\u00e1c v\u00e0 logic x\u00e1c th\u1ef1c.<\/p>\n<h2>Chuy\u1ec3n \u0111\u1ed5i thi\u1ebft k\u1ebf sang tri\u1ec3n khai \ud83d\udee0\ufe0f<\/h2>\n<p>Sau khi s\u01a1 \u0111\u1ed3 l\u1edbp \u0111\u01b0\u1ee3c ho\u00e0n thi\u1ec7n, vi\u1ec7c chuy\u1ec3n \u0111\u1ed5i sang m\u00e3 th\u1ef1c t\u1ebf v\u00e0 l\u01b0u tr\u1eef \u0111\u00f2i h\u1ecfi c\u00e1c chi\u1ebfn l\u01b0\u1ee3c c\u1ee5 th\u1ec3 cho t\u1eebng lo\u1ea1i m\u1ed1i quan h\u1ec7.<\/p>\n<h3>Thi\u1ebft k\u1ebf l\u01b0\u1ee3c \u0111\u1ed3 c\u01a1 s\u1edf d\u1eef li\u1ec7u<\/h3>\n<p>L\u01b0\u1ee3c \u0111\u1ed3 v\u1eadt l\u00fd l\u00e0 ph\u1ea7n c\u1ee9ng nh\u1eafc nh\u1ea5t c\u1ee7a h\u1ec7 th\u1ed1ng. Nh\u1eefng thay \u0111\u1ed5i \u1edf \u0111\u00e2y t\u1ed1n k\u00e9m.<\/p>\n<ul>\n<li><strong>Chu\u1ea9n h\u00f3a:<\/strong>\u0110\u1ea3m b\u1ea3o thi\u1ebft k\u1ebf c\u1ee7a b\u1ea1n tu\u00e2n theo c\u00e1c quy t\u1eafc chu\u1ea9n h\u00f3a (th\u01b0\u1eddng \u0111\u1ebfn 3NF). D\u1eef li\u1ec7u tr\u00f9ng l\u1eb7p th\u01b0\u1eddng xu\u1ea5t ph\u00e1t t\u1eeb vi\u1ec7c hi\u1ec3u sai c\u00e1c m\u1ed1i quan h\u1ec7.<\/li>\n<li><strong>Ch\u1ec9 m\u1ee5c:<\/strong>C\u00e1c c\u1ed9t kh\u00f3a ngo\u1ea1i n\u00ean \u0111\u01b0\u1ee3c t\u1ea1o ch\u1ec9 m\u1ee5c. \u0110i\u1ec1u n\u00e0y gi\u00fap t\u0103ng t\u1ed1c \u0111\u00e1ng k\u1ec3 c\u00e1c thao t\u00e1c n\u1ed1i b\u1ea3ng v\u00e0 ki\u1ec3m tra r\u00e0ng bu\u1ed9c.<\/li>\n<li><strong>Ki\u1ec3u d\u1eef li\u1ec7u:<\/strong> \u0110\u1ea3m b\u1ea3o ki\u1ec3u d\u1eef li\u1ec7u c\u1ee7a kh\u00f3a ch\u00ednh kh\u1edbp ch\u00ednh x\u00e1c v\u1edbi kh\u00f3a ngo\u1ea1i. C\u00e1c ki\u1ec3u kh\u00f4ng kh\u1edbp s\u1ebd d\u1eabn \u0111\u1ebfn l\u1ed7i th\u1eddi gian ch\u1ea1y.<\/li>\n<\/ul>\n<h3>Logic l\u1edbp \u1ee9ng d\u1ee5ng<\/h3>\n<p>L\u1edbp m\u00e3 l\u00e0 n\u01a1i c\u00e1c quy t\u1eafc kinh doanh th\u1ef1c thi m\u1ed1i quan h\u1ec7.<\/p>\n<ul>\n<li><strong>X\u00e1c th\u1ef1c:<\/strong> Tr\u01b0\u1edbc khi l\u01b0u m\u1ed9t \u0111\u1ed1i t\u01b0\u1ee3ng, h\u00e3y x\u00e1c th\u1ef1c xem c\u00e1c r\u00e0ng bu\u1ed9c m\u1ed1i quan h\u1ec7 c\u00f3 \u0111\u01b0\u1ee3c \u0111\u00e1p \u1ee9ng hay kh\u00f4ng. V\u00ed d\u1ee5, kh\u00f4ng cho ph\u00e9p m\u1ed9t sinh vi\u00ean \u0111\u0103ng k\u00fd v\u00e0o m\u1ed9t kh\u00f3a h\u1ecdc \u0111\u00e3 \u0111\u1ea7y.<\/li>\n<li><strong>Qu\u1ea3n l\u00fd giao d\u1ecbch:<\/strong> Khi t\u1ea1o ho\u1eb7c c\u1eadp nh\u1eadt c\u00e1c \u0111\u1ed1i t\u01b0\u1ee3ng li\u00ean quan, bao b\u1ecdc c\u00e1c thao t\u00e1c trong c\u00e1c giao d\u1ecbch. \u0110i\u1ec1u n\u00e0y \u0111\u1ea3m b\u1ea3o r\u1eb1ng n\u1ebfu m\u1ed9t ph\u1ea7n c\u1ee7a m\u1ed1i quan h\u1ec7 th\u1ea5t b\u1ea1i, to\u00e0n b\u1ed9 thay \u0111\u1ed5i s\u1ebd \u0111\u01b0\u1ee3c ho\u00e0n t\u00e1c.<\/li>\n<li><strong>Ph\u1ea3n h\u1ed3i API:<\/strong> Khi c\u00f4ng khai d\u1eef li\u1ec7u qua API, h\u00e3y quy\u1ebft \u0111\u1ecbnh m\u1ee9c \u0111\u1ed9 l\u1ed3ng gh\u00e9p c\u00e1c \u0111\u1ed1i t\u01b0\u1ee3ng li\u00ean quan. Tr\u1ea3 v\u1ec1 m\u1ed9t \u0111\u1ed1i t\u01b0\u1ee3ng Kh\u00e1ch h\u00e0ng \u0111\u1ea7y \u0111\u1ee7 c\u00f9ng t\u1ea5t c\u1ea3 c\u00e1c \u0110\u01a1n h\u00e0ng c\u1ee7a h\u1ecd trong m\u1ed9t ph\u1ea3n h\u1ed3i duy nh\u1ea5t c\u00f3 th\u1ec3 d\u1eabn \u0111\u1ebfn \u0111i\u1ec3m ngh\u1ebdn hi\u1ec7u su\u1ea5t.<\/li>\n<\/ul>\n<h2>Nh\u1eefng sai l\u1ea7m ph\u1ed5 bi\u1ebfn v\u00e0 m\u1eabu ch\u1ed1ng l\u1ea1i t\u1ed1t \ud83d\udeab<\/h2>\n<p>Ngay c\u1ea3 nh\u1eefng nh\u00e0 thi\u1ebft k\u1ebf c\u00f3 kinh nghi\u1ec7m c\u0169ng m\u1eafc sai l\u1ea7m khi x\u00e1c \u0111\u1ecbnh t\u00ednh \u0111a d\u1ea1ng. Nh\u1eadn di\u1ec7n nh\u1eefng m\u1eabu n\u00e0y s\u1edbm s\u1ebd ti\u1ebft ki\u1ec7m r\u1ea5t nhi\u1ec1u th\u1eddi gian t\u00e1i c\u1ea5u tr\u00fac sau n\u00e0y.<\/p>\n<ul>\n<li><strong>Gi\u1ea3 \u0111\u1ecbnh r\u1eb1ng N:N lu\u00f4n c\u1ea7n thi\u1ebft:<\/strong> N\u1ebfu hai th\u1ef1c th\u1ec3 d\u01b0\u1eddng nh\u01b0 c\u00f3 li\u00ean h\u1ec7, h\u00e3y ki\u1ec3m tra xem ch\u00fang th\u1ef1c s\u1ef1 c\u00f3 c\u1ea7n m\u1ed9t li\u00ean k\u1ebft tr\u1ef1c ti\u1ebfp hay kh\u00f4ng. Th\u01b0\u1eddng th\u00ec m\u1ed1i quan h\u1ec7 1:N l\u00e0 \u0111\u1ee7 n\u1ebfu m\u1ed1i quan h\u1ec7 mang t\u00ednh h\u01b0\u1edbng.<\/li>\n<li><strong>B\u1ecf qua t\u00ednh t\u00f9y ch\u1ecdn:<\/strong>Thi\u1ebft k\u1ebf m\u1ed9t li\u00ean k\u1ebft b\u1eaft bu\u1ed9c (1..1) khi m\u1ed1i quan h\u1ec7 th\u1ef1c t\u1ebf l\u00e0 t\u00f9y ch\u1ecdn (0..1) s\u1ebd d\u1eabn \u0111\u1ebfn l\u1ed7i nh\u1eadp d\u1eef li\u1ec7u v\u00e0 c\u00e1c h\u1ec7 th\u1ed1ng c\u1ee9ng nh\u1eafc.<\/li>\n<li><strong>Ph\u1ee5 thu\u1ed9c v\u00f2ng:<\/strong> Khi Class A tham chi\u1ebfu \u0111\u1ebfn Class B, v\u00e0 Class B tham chi\u1ebfu ng\u01b0\u1ee3c l\u1ea1i Class A, vi\u1ec7c tu\u1ea7n t\u1ef1 h\u00f3a v\u00e0 qu\u1ea3n l\u00fd b\u1ed9 nh\u1edb c\u00f3 th\u1ec3 tr\u1edf n\u00ean ph\u1ee9c t\u1ea1p. H\u00e3y c\u1ea9n tr\u1ecdng v\u1edbi \u0111\u1ec7 quy s\u00e2u trong c\u00e1c thu\u1eadt to\u00e1n duy\u1ec7t.<\/li>\n<li><strong>B\u1ea3ng giao nhau b\u1ecb qu\u00e1 thi\u1ebft k\u1ebf:<\/strong> \u0110\u1eebng t\u1ea1o b\u1ea3ng giao nhau n\u1ebfu m\u1ed1i quan h\u1ec7 \u0111\u01a1n gi\u1ea3n v\u00e0 kh\u00f4ng c\u1ea7n thu\u1ed9c t\u00ednh ri\u00eang. \u0110\u00f4i khi ch\u1ec9 c\u1ea7n m\u1ed9t kh\u00f3a ngo\u1ea1i duy nh\u1ea5t l\u00e0 \u0111\u1ee7.<\/li>\n<\/ul>\n<h2>So s\u00e1nh c\u00e1c lo\u1ea1i m\u1ed1i quan h\u1ec7 \ud83d\udcca<\/h2>\n<p>\u0110\u1ec3 t\u00f3m t\u1eaft s\u1ef1 kh\u00e1c bi\u1ec7t v\u00e0 c\u00e1c th\u1ecfa thu\u1eadn, h\u00e3y tham kh\u1ea3o t\u1ed5ng quan v\u1ec1 ba lo\u1ea1i c\u1ea5p \u0111\u1ed9 ch\u00ednh sau.<\/p>\n<table>\n<thead>\n<tr>\n<th>T\u00ednh n\u0103ng<\/th>\n<th>M\u1ed9t-\u0111\u1ed1i-m\u1ed9t (1:1)<\/th>\n<th>M\u1ed9t-\u0111\u1ed1i-nhi\u1ec1u (1:N)<\/th>\n<th>Nhi\u1ec1u-\u0111\u1ed1i-nhi\u1ec1u (N:N)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td><strong>K\u00fd hi\u1ec7u<\/strong><\/td>\n<td>1 \u2014 1<\/td>\n<td>1 \u2014 *<\/td>\n<td>* \u2014 *<\/td>\n<\/tr>\n<tr>\n<td><strong>Tri\u1ec3n khai C\u01a1 s\u1edf d\u1eef li\u1ec7u<\/strong><\/td>\n<td>Kh\u00f3a ngo\u1ea1i v\u1edbi r\u00e0ng bu\u1ed9c duy nh\u1ea5t<\/td>\n<td>Kh\u00f3a ngo\u1ea1i trong b\u1ea3ng con<\/td>\n<td>B\u1ea3ng li\u00ean k\u1ebft (Th\u1ef1c th\u1ec3 li\u00ean k\u1ebft)<\/td>\n<\/tr>\n<tr>\n<td><strong>C\u1ea5u tr\u00fac m\u00e3 ngu\u1ed3n<\/strong><\/td>\n<td>Tham chi\u1ebfu \u0111\u1ebfn m\u1ed9t \u0111\u1ed1i t\u01b0\u1ee3ng duy nh\u1ea5t<\/td>\n<td>B\u1ed9 s\u01b0u t\u1eadp\/Danh s\u00e1ch c\u00e1c \u0111\u1ed1i t\u01b0\u1ee3ng<\/td>\n<td>B\u1ed9 s\u01b0u t\u1eadp c\u00e1c b\u1ed9 s\u01b0u t\u1eadp<\/td>\n<\/tr>\n<tr>\n<td><strong>\u0110\u1ed9 ph\u1ee9c t\u1ea1p truy v\u1ea5n<\/strong><\/td>\n<td>Th\u1ea5p<\/td>\n<td>Trung b\u00ecnh<\/td>\n<td>Cao (Y\u00eau c\u1ea7u k\u1ebft n\u1ed1i)<\/td>\n<\/tr>\n<tr>\n<td><strong>T\u00ednh linh ho\u1ea1t<\/strong><\/td>\n<td>Th\u1ea5p (Nghi\u00eam ng\u1eb7t)<\/td>\n<td>Cao<\/td>\n<td>R\u1ea5t cao<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2>Nh\u1eefng c\u00e2n nh\u1eafc cu\u1ed1i c\u00f9ng v\u1ec1 t\u00ednh to\u00e0n v\u1eb9n d\u1eef li\u1ec7u \u2705<\/h2>\n<p>S\u1ef1 \u1ed5n \u0111\u1ecbnh c\u1ee7a m\u1ed9t h\u1ec7 th\u1ed1ng ph\u1ea7n m\u1ec1m ph\u1ee5 thu\u1ed9c r\u1ea5t nhi\u1ec1u v\u00e0o t\u00ednh ch\u00ednh x\u00e1c c\u1ee7a c\u00e1c m\u1ed1i quan h\u1ec7 c\u1ee7a n\u00f3. Khi x\u00e1c \u0111\u1ecbnh t\u00ednh \u0111a d\u1ea1ng, b\u1ea1n \u0111ang thi\u1ebft l\u1eadp c\u00e1c quy t\u1eafc t\u01b0\u01a1ng t\u00e1c cho d\u1eef li\u1ec7u c\u1ee7a m\u00ecnh. M\u1ed9t s\u01a1 \u0111\u1ed3 l\u1edbp \u0111\u01b0\u1ee3c x\u00e1c \u0111\u1ecbnh r\u00f5 s\u1ebd ho\u1ea1t \u0111\u1ed9ng nh\u01b0 m\u1ed9t b\u1ea3n v\u1ebd thi\u1ebft k\u1ebf, gi\u00fap \u0111\u1ed3ng b\u1ed9 h\u00f3a c\u01a1 s\u1edf d\u1eef li\u1ec7u, m\u00e3 ngu\u1ed3n v\u00e0 logic kinh doanh.<\/p>\n<p>Lu\u00f4n ki\u1ec3m tra c\u00e1c gi\u1ea3 \u0111\u1ecbnh c\u1ee7a b\u1ea1n. V\u1ebd s\u01a1 \u0111\u1ed3, tri\u1ec3n khai m\u1ed9t b\u1ea3n th\u1eed nghi\u1ec7m v\u00e0 ki\u1ec3m tra xem d\u1eef li\u1ec7u c\u00f3 ch\u1ea3y m\u1ed9t c\u00e1ch t\u1ef1 nhi\u00ean hay kh\u00f4ng. N\u1ebfu b\u1ea1n li\u00ean t\u1ee5c ph\u1ea3i th\u00eam c\u00e1c bi\u1ec7n ph\u00e1p kh\u1eafc ph\u1ee5c \u0111\u1ec3 \u00e9p d\u1eef li\u1ec7u v\u00e0o c\u1ea5u tr\u00fac 1:N m\u00e0 c\u1ea3m gi\u00e1c nh\u01b0 N:N, th\u00ec \u0111\u00e3 \u0111\u1ebfn l\u00fac xem x\u00e9t l\u1ea1i thi\u1ebft k\u1ebf.<\/p>\n<p>B\u1eb1ng c\u00e1ch tu\u00e2n th\u1ee7 c\u00e1c nguy\u00ean t\u1eafc n\u00e0y, b\u1ea1n \u0111\u1ea3m b\u1ea3o h\u1ec7 th\u1ed1ng c\u1ee7a m\u00ecnh v\u1eabn c\u00f3 th\u1ec3 m\u1edf r\u1ed9ng, d\u1ec5 b\u1ea3o tr\u00ec v\u00e0 nh\u1ea5t qu\u00e1n v\u1ec1 m\u1eb7t logic. C\u00f4ng s\u1ee9c b\u1ecf ra \u0111\u1ec3 x\u00e1c \u0111\u1ecbnh ch\u00ednh x\u00e1c c\u00e1c m\u1ed1i quan h\u1ec7 1:1, 1:N v\u00e0 N:N s\u1ebd mang l\u1ea1i l\u1ee3i \u00edch r\u00f5 r\u1ec7t trong vi\u1ec7c gi\u1ea3m l\u1ed7i v\u00e0 c\u1ea5u tr\u00fac m\u00e3 ngu\u1ed3n r\u00f5 r\u00e0ng h\u01a1n trong su\u1ed1t v\u00f2ng \u0111\u1eddi d\u1ef1 \u00e1n.<\/p>\n<h3>Nh\u1eefng \u0111i\u1ec3m ch\u00ednh c\u1ea7n l\u01b0u \u00fd<\/h3>\n<ul>\n<li><strong>K\u00fd hi\u1ec7u c\u00f3 \u00fd ngh\u0129a:<\/strong>S\u1eed d\u1ee5ng c\u00e1c k\u00fd hi\u1ec7u chu\u1ea9n (1, 0..1, *) \u0111\u1ec3 truy\u1ec1n \u0111\u1ea1t m\u1ee5c \u0111\u00edch m\u1ed9t c\u00e1ch r\u00f5 r\u00e0ng.<\/li>\n<li><strong>Ph\u00f9 h\u1ee3p v\u1edbi c\u01a1 s\u1edf d\u1eef li\u1ec7u:<\/strong>\u0110\u1ea3m b\u1ea3o l\u01b0\u1ee3c \u0111\u1ed3 c\u1ee7a b\u1ea1n h\u1ed7 tr\u1ee3 s\u01a1 \u0111\u1ed3 m\u00e0 kh\u00f4ng bu\u1ed9c ph\u1ea3i d\u00f9ng c\u00e1c bi\u1ec7n ph\u00e1p kh\u1eafc ph\u1ee5c kh\u00f3 ch\u1ecbu.<\/li>\n<li><strong>T\u00ednh t\u00f9y ch\u1ecdn l\u00e0 then ch\u1ed1t:<\/strong>Ph\u00e2n bi\u1ec7t gi\u1eefa \u201cph\u1ea3i t\u1ed3n t\u1ea1i\u201d v\u00e0 \u201cc\u00f3 th\u1ec3 t\u1ed3n t\u1ea1i\u201d \u0111\u1ec3 tr\u00e1nh c\u00e1c r\u00e0ng bu\u1ed9c c\u1ee9ng nh\u1eafc.<\/li>\n<li><strong>Qu\u1ea3n l\u00fd \u0111\u1ed9 ph\u1ee9c t\u1ea1p:<\/strong>S\u1eed d\u1ee5ng b\u1ea3ng li\u00ean k\u1ebft cho c\u00e1c m\u1ed1i quan h\u1ec7 N:N \u0111\u1ec3 duy tr\u00ec t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu.<\/li>\n<li><strong>Ki\u1ec3m tra s\u1edbm:<\/strong> Ki\u1ec3m tra c\u00e1c m\u1ed1i quan h\u1ec7 trong giai \u0111o\u1ea1n thi\u1ebft k\u1ebf \u0111\u1ec3 ng\u0103n ng\u1eeba n\u1ee3 ki\u1ebfn tr\u00fac.<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Trong b\u1ed1i c\u1ea3nh ki\u1ebfn tr\u00fac ph\u1ea7n m\u1ec1m v\u00e0 m\u00f4 h\u00ecnh h\u00f3a d\u1eef li\u1ec7u, \u00edt kh\u00e1i ni\u1ec7m n\u00e0o mang tr\u1ecdng l\u01b0\u1ee3ng b\u1eb1ng c\u00e1c m\u1ed1i quan h\u1ec7 gi\u1eefa c\u00e1c th\u1ef1c th\u1ec3. Khi&hellip;<\/p>\n","protected":false},"author":1,"featured_media":1121,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_yoast_wpseo_title":"H\u01b0\u1edbng d\u1eabnMultiplicity s\u01a1 \u0111\u1ed3 l\u1edbp: 1:1, 1:N, N:N \u0111\u01b0\u1ee3c gi\u1ea3i th\u00edch","_yoast_wpseo_metadesc":"M\u1ed9t h\u01b0\u1edbng d\u1eabn to\u00e0n di\u1ec7n \u0111\u1ec3 hi\u1ec3uMultiplicity s\u01a1 \u0111\u1ed3 l\u1edbp. H\u1ecdc c\u00e1ch tri\u1ec3n khai c\u00e1c m\u1ed1i quan h\u1ec7 1:1, 1:N v\u00e0 N:N trong m\u00f4 h\u00ecnh d\u1eef li\u1ec7u v\u00e0 thi\u1ebft k\u1ebf ph\u1ea7n m\u1ec1m.","fifu_image_url":"","fifu_image_alt":"","footnotes":""},"categories":[13],"tags":[43,45],"class_list":["post-1120","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uml","tag-academic","tag-class-diagram"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.1.1 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>H\u01b0\u1edbng d\u1eabnMultiplicity s\u01a1 \u0111\u1ed3 l\u1edbp: 1:1, 1:N, N:N \u0111\u01b0\u1ee3c gi\u1ea3i th\u00edch<\/title>\n<meta name=\"description\" content=\"M\u1ed9t h\u01b0\u1edbng d\u1eabn to\u00e0n di\u1ec7n \u0111\u1ec3 hi\u1ec3uMultiplicity s\u01a1 \u0111\u1ed3 l\u1edbp. 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