{"id":61,"date":"2026-04-05T18:03:13","date_gmt":"2026-04-05T18:03:13","guid":{"rendered":"https:\/\/www.we-notes.com\/vn\/er-diagram-relations-keys-cardinality-guide\/"},"modified":"2026-04-05T18:03:13","modified_gmt":"2026-04-05T18:03:13","slug":"er-diagram-relations-keys-cardinality-guide","status":"publish","type":"post","link":"https:\/\/www.we-notes.com\/vn\/er-diagram-relations-keys-cardinality-guide\/","title":{"rendered":"C\u00e2u h\u1ecfi & C\u00e2u tr\u1ea3 l\u1eddi: Gi\u1ea3i quy\u1ebft 15 nh\u1ea7m l\u1eabn ph\u1ed5 bi\u1ebfn v\u1ec1 m\u1ed1i quan h\u1ec7, kh\u00f3a v\u00e0 t\u00ednh cardinality trong s\u01a1 \u0111\u1ed3 ER"},"content":{"rendered":"

S\u01a1 \u0111\u1ed3 quan h\u1ec7 th\u1ef1c th\u1ec3 (ERD) \u0111\u00f3ng vai tr\u00f2 nh\u01b0 b\u1ea3n v\u1ebd thi\u1ebft k\u1ebf cho ki\u1ebfn tr\u00fac c\u01a1 s\u1edf d\u1eef li\u1ec7u. Tuy nhi\u00ean, ngay c\u1ea3 nh\u1eefng nh\u00e0 thi\u1ebft k\u1ebf c\u00f3 kinh nghi\u1ec7m c\u0169ng g\u1eb7p kh\u00f3 kh\u0103n khi chuy\u1ec3n \u0111\u1ed5i logic kinh doanh th\u00e0nh m\u00f4 h\u00ecnh d\u1eef li\u1ec7u. S\u1ef1 m\u01a1 h\u1ed3 th\u01b0\u1eddng xu\u1ea5t ph\u00e1t t\u1eeb s\u1ef1 tr\u00f9ng l\u1eb7p thu\u1eadt ng\u1eef v\u00e0 nh\u1eefng kh\u00e1c bi\u1ec7t tinh t\u1ebf gi\u1eefa c\u00e1c th\u00e0nh ph\u1ea7n c\u1ea5u tr\u00fac. H\u01b0\u1edbng d\u1eabn n\u00e0y gi\u1ea3i quy\u1ebft nh\u1eefng c\u00e2u h\u1ecfi ph\u1ed5 bi\u1ebfn nh\u1ea5t v\u1ec1 kh\u00f3a, t\u00ednh cardinality v\u00e0 m\u1ed1i quan h\u1ec7.<\/p>\n

Hi\u1ec3u r\u00f5 nh\u1eefng kh\u00e1i ni\u1ec7m n\u00e0y gi\u00fap ng\u0103n ng\u1eeba s\u1ef1 tr\u00f9ng l\u1eb7p d\u1eef li\u1ec7u v\u00e0 \u0111\u1ea3m b\u1ea3o hi\u1ec7u su\u1ea5t truy v\u1ea5n. Ch\u00fang ta s\u1ebd \u0111i qua 15 \u0111i\u1ec3m g\u00e2y nh\u1ea7m l\u1eabn c\u1ee5 th\u1ec3, ph\u00e2n t\u00edch ch\u00fang th\u00e0nh nh\u1eefng \u0111\u1ecbnh ngh\u0129a r\u00f5 r\u00e0ng v\u00e0 c\u00f3 th\u1ec3 th\u1ef1c hi\u1ec7n \u0111\u01b0\u1ee3c. M\u1ed7i ph\u1ea7n bao g\u1ed3m c\u00e1c v\u00ed d\u1ee5 th\u1ef1c t\u1ebf v\u00e0 m\u00f4 t\u1ea3 tr\u1ef1c quan \u0111\u1ec3 l\u00e0m r\u00f5 c\u01a1 ch\u1ebf ho\u1ea1t \u0111\u1ed9ng b\u00ean trong.<\/p>\n

\n
\"Chalkboard-style<\/figure>\n<\/div>\n

1. S\u1ef1 kh\u00e1c bi\u1ec7t gi\u1eefa m\u1ed9t Th\u1ef1c th\u1ec3 v\u00e0 m\u1ed9t Thu\u1ed9c t\u00ednh l\u00e0 g\u00ec? \ud83c\udff7\ufe0f<\/h2>\n

M\u1ed9t Th\u1ef1c th\u1ec3<\/strong>\u0111\u1ea1i di\u1ec7n cho m\u1ed9t \u0111\u1ed1i t\u01b0\u1ee3ng ho\u1eb7c kh\u00e1i ni\u1ec7m trong th\u1ebf gi\u1edbi th\u1ef1c m\u00e0 d\u1eef li\u1ec7u \u0111\u01b0\u1ee3c l\u01b0u tr\u1eef. N\u00f3 th\u01b0\u1eddng \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n d\u01b0\u1edbi d\u1ea1ng h\u00ecnh ch\u1eef nh\u1eadt. C\u00e1c v\u00ed d\u1ee5 bao g\u1ed3m Kh\u00e1ch h\u00e0ng<\/em>, S\u1ea3n ph\u1ea9m<\/em>, ho\u1eb7c \u0110\u01a1n h\u00e0ng<\/em>.<\/p>\n

M\u1ed9t Thu\u1ed9c t\u00ednh<\/strong>m\u00f4 t\u1ea3 m\u1ed9t thu\u1ed9c t\u00ednh c\u1ee7a m\u1ed9t th\u1ef1c th\u1ec3. N\u00f3 \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n d\u01b0\u1edbi d\u1ea1ng h\u00ecnh elip k\u1ebft n\u1ed1i v\u1edbi th\u1ef1c th\u1ec3. V\u00ed d\u1ee5, T\u00eanKh\u00e1chH\u00e0ng<\/em> ho\u1eb7c Gi\u00e1S\u1ea3nPh\u1ea9m<\/em>l\u00e0 c\u00e1c thu\u1ed9c t\u00ednh c\u1ee7a c\u00e1c th\u1ef1c th\u1ec3 \u0111\u01b0\u1ee3c n\u00eau \u1edf tr\u00ean.<\/p>\n

    \n
  • Th\u1ef1c th\u1ec3:<\/strong> Danh t\u1eeb (Ai\/\u0110i\u1ec1u g\u00ec).<\/li>\n
  • Thu\u1ed9c t\u00ednh:<\/strong> T\u00ednh t\u1eeb (M\u00f4 t\u1ea3 n\u00f3 nh\u01b0 th\u1ebf n\u00e0o).<\/li>\n<\/ul>\n

    S\u1ef1 nh\u1ea7m l\u1eabn th\u01b0\u1eddng x\u1ea3y ra khi m\u1ed9t thu\u1ed9c t\u00ednh ch\u1ee9a nhi\u1ec1u th\u00f4ng tin kh\u00e1c nhau. N\u1ebfu \u0110\u1ecba ch\u1ec9<\/em>l\u00e0 m\u1ed9t thu\u1ed9c t\u00ednh, c\u00f3 th\u1ec3 s\u1ebd t\u1ed1t h\u01a1n n\u1ebfu chia n\u00f3 th\u00e0nh \u0110\u01b0\u1eddng<\/em>, Th\u00e0nh ph\u1ed1<\/em>, v\u00e0 B\u01b0u ch\u00ednh<\/em> \u0111\u1ec3 chu\u1ea9n h\u00f3a t\u1ed1t h\u01a1n.<\/p>\n

    2. Kh\u00f3a ch\u00ednh kh\u00e1c v\u1edbi kh\u00f3a duy nh\u1ea5t nh\u01b0 th\u1ebf n\u00e0o? \ud83d\udd11<\/h2>\n

    C\u1ea3 hai \u0111\u1ec1u \u0111\u1ea3m b\u1ea3o t\u00ednh to\u00e0n v\u1eb9n d\u1eef li\u1ec7u, nh\u01b0ng c\u00e1ch s\u1eed d\u1ee5ng c\u1ee7a ch\u00fang kh\u00e1c nhau.<\/p>\n

      \n
    • Kh\u00f3a ch\u00ednh:<\/strong> X\u00e1c \u0111\u1ecbnh duy nh\u1ea5t m\u1ed7i h\u00e0ng trong m\u1ed9t b\u1ea3ng. M\u1ed9t b\u1ea3ng ch\u1ec9 c\u00f3 th\u1ec3 c\u00f3 m\u1ed9t kh\u00f3a ch\u00ednh. N\u00f3 kh\u00f4ng th\u1ec3 ch\u1ee9a gi\u00e1 tr\u1ecb null.<\/li>\n
    • Kh\u00f3a duy nh\u1ea5t:<\/strong> \u0110\u1ea3m b\u1ea3o t\u1ea5t c\u1ea3 c\u00e1c gi\u00e1 tr\u1ecb trong m\u1ed9t c\u1ed9t l\u00e0 kh\u00e1c nhau. M\u1ed9t b\u1ea3ng c\u00f3 th\u1ec3 c\u00f3 nhi\u1ec1u kh\u00f3a duy nh\u1ea5t. Gi\u00e1 tr\u1ecb null th\u01b0\u1eddng \u0111\u01b0\u1ee3c ph\u00e9p (ph\u1ee5 thu\u1ed9c v\u00e0o c\u00e1ch tri\u1ec3n khai).<\/li>\n<\/ul>\n

      H\u00e3y h\u00ecnh dung kh\u00f3a ch\u00ednh nh\u01b0 s\u1ed1 b\u1ea3o hi\u1ec3m x\u00e3 h\u1ed9i cho m\u1ed9t b\u1ea3n ghi. Kh\u00f3a duy nh\u1ea5t gi\u1ed1ng nh\u01b0 s\u1ed1 h\u1ed9 chi\u1ebfu\u2014c\u0169ng duy nh\u1ea5t, nh\u01b0ng b\u1ea1n c\u00f3 th\u1ec3 c\u00f3 nhi\u1ec1u h\u01a1n m\u1ed9t \u0111\u1ecbnh danh duy nh\u1ea5t cho m\u1ed9t ng\u01b0\u1eddi.<\/p>\n

      3. Kh\u00f3a ngo\u1ea1i l\u00e0 g\u00ec v\u00e0 n\u00f3 li\u00ean k\u1ebft c\u00e1c b\u1ea3ng nh\u01b0 th\u1ebf n\u00e0o? \ud83d\udd17<\/h2>\n

      M\u1ed9t kh\u00f3a ngo\u1ea1i<\/strong> l\u00e0 m\u1ed9t tr\u01b0\u1eddng (ho\u1eb7c t\u1eadp h\u1ee3p c\u00e1c tr\u01b0\u1eddng) trong m\u1ed9t b\u1ea3ng tham chi\u1ebfu \u0111\u1ebfn kh\u00f3a ch\u00ednh trong b\u1ea3ng kh\u00e1c. N\u00f3 thi\u1ebft l\u1eadp m\u1ed1i li\u00ean k\u1ebft gi\u1eefa hai b\u1ea3ng.<\/p>\n

      H\u00e3y xem x\u00e9t m\u1ed9t b\u1ea3ng \u0110\u01a1n h\u00e0ng<\/em> b\u1ea3ng. N\u00f3 c\u1ea7n bi\u1ebft ai \u0111\u00e3 \u0111\u1eb7t \u0111\u01a1n h\u00e0ng. Tr\u01b0\u1eddng Kh\u00e1ch h\u00e0ng<\/em> \u0111\u00e3 \u0111\u1eb7t \u0111\u01a1n h\u00e0ng. Tr\u01b0\u1eddng CustomerID<\/em> trong b\u1ea3ng \u0110\u01a1n h\u00e0ng<\/em> l\u00e0 kh\u00f3a ngo\u1ea1i.<\/p>\n\n\n\n\n\n\n
      B\u1ea3ng<\/th>\nC\u1ed9t<\/th>\nVai tr\u00f2<\/th>\n<\/tr>\n<\/thead>\n
      Kh\u00e1ch h\u00e0ng<\/td>\nCustomerID<\/td>\nKh\u00f3a ch\u00ednh<\/td>\n<\/tr>\n
      \u0110\u01a1n h\u00e0ng<\/td>\nCustomerID<\/td>\nKh\u00f3a ngo\u1ea1i<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

      M\u1ed1i quan h\u1ec7 n\u00e0y cho ph\u00e9p c\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u1ea3m b\u1ea3o t\u00ednh to\u00e0n v\u1eb9n tham chi\u1ebfu, \u0111\u1ea3m b\u1ea3o r\u1eb1ng kh\u00f4ng c\u00f3 \u0111\u01a1n h\u00e0ng n\u00e0o t\u1ed3n t\u1ea1i m\u00e0 kh\u00f4ng c\u00f3 kh\u00e1ch h\u00e0ng h\u1ee3p l\u1ec7.<\/p>\n

      4. Khi n\u00e0o th\u00ec m\u1ed1i quan h\u1ec7 l\u00e0 M\u1ed9t-\u0111\u1ed1i-m\u1ed9t? \ud83e\udd1d<\/h2>\n

      M\u1ed1i quan h\u1ec7 M\u1ed9t-\u0111\u1ed1i-m\u1ed9t (1:1) x\u1ea3y ra khi m\u1ed9t b\u1ea3n ghi duy nh\u1ea5t trong B\u1ea3ng A li\u00ean k\u1ebft v\u1edbi \u0111\u00fang m\u1ed9t b\u1ea3n ghi trong B\u1ea3ng B, v\u00e0 ng\u01b0\u1ee3c l\u1ea1i.<\/p>\n

        \n
      • V\u00ed d\u1ee5:<\/strong> M\u1ed9t Ng\u01b0\u1eddi<\/em> v\u00e0 m\u1ed9t H\u1ed9 chi\u1ebfu<\/em>.<\/li>\n
      • Tri\u1ec3n khai:<\/strong>Th\u01b0\u1eddng \u0111\u01b0\u1ee3c tri\u1ec3n khai b\u1eb1ng c\u00e1ch \u0111\u1eb7t Kh\u00f3a ch\u00ednh c\u1ee7a m\u1ed9t b\u1ea3ng l\u00e0m Kh\u00f3a ngo\u1ea1i trong b\u1ea3ng kia.<\/li>\n<\/ul>\n

        \u0110i\u1ec1u n\u00e0y ph\u1ed5 bi\u1ebfn khi t\u00e1ch m\u1ed9t th\u1ef1c th\u1ec3 \u0111\u1ec3 t\u1ed1i \u01b0u hi\u1ec7u su\u1ea5t ho\u1eb7c b\u1ea3o m\u1eadt. V\u00ed d\u1ee5, di chuy\u1ec3n d\u1eef li\u1ec7u nh\u1ea1y c\u1ea3m nh\u01b0 S\u1ed1 an sinh x\u00e3 h\u1ed9i<\/em> v\u00e0o m\u1ed9t b\u1ea3ng ri\u00eang bi\u1ec7t \u0111\u01b0\u1ee3c li\u00ean k\u1ebft theo ki\u1ec3u 1:1.<\/p>\n

        5. M\u1ed1i quan h\u1ec7 M\u1ed9t-\u0111\u1ed1i-nhi\u1ec1u ho\u1ea1t \u0111\u1ed9ng nh\u01b0 th\u1ebf n\u00e0o? \ud83d\udce6<\/h2>\n

        \u0110\u00e2y l\u00e0 ki\u1ec3u m\u1ed1i quan h\u1ec7 ph\u1ed5 bi\u1ebfn nh\u1ea5t. M\u1ed9t b\u1ea3n ghi duy nh\u1ea5t trong B\u1ea3ng A li\u00ean k\u1ebft v\u1edbi nhi\u1ec1u b\u1ea3n ghi trong B\u1ea3ng B, nh\u01b0ng m\u1ed9t b\u1ea3n ghi trong B\u1ea3ng B ch\u1ec9 li\u00ean k\u1ebft v\u1edbi m\u1ed9t b\u1ea3n ghi duy nh\u1ea5t trong B\u1ea3ng A.<\/p>\n

          \n
        • V\u00ed d\u1ee5:<\/strong> Ph\u00f2ng ban<\/em> \u0111\u1ebfn Nh\u00e2n vi\u00ean<\/em>.<\/li>\n
        • H\u01b0\u1edbng:<\/strong>M\u1ed9t Ph\u00f2ng ban c\u00f3 nhi\u1ec1u Nh\u00e2n vi\u00ean.<\/li>\n<\/ul>\n

          Trong s\u01a1 \u0111\u1ed3 ERD, \u0111i\u1ec1u n\u00e0y \u0111\u01b0\u1ee3c v\u1ebd b\u1eb1ng m\u1ed9t \u0111\u01b0\u1eddng n\u1ed1i hai th\u1ef1c th\u1ec3. B\u00ean c\u00f3 \u201cNhi\u1ec1u\u201d s\u1ebd nh\u1eadn Kh\u00f3a ngo\u1ea1i.<\/p>\n

          6. T\u1ea1i sao m\u1ed1i quan h\u1ec7 Nhi\u1ec1u-\u0111\u1ed1i-nhi\u1ec1u l\u1ea1i g\u00e2y kh\u00f3 kh\u0103n? \u2696\ufe0f<\/h2>\n

          M\u1ed1i quan h\u1ec7 Nhi\u1ec1u-\u0111\u1ed1i-nhi\u1ec1u (M:N) t\u1ed3n t\u1ea1i khi nhi\u1ec1u b\u1ea3n ghi trong B\u1ea3ng A li\u00ean k\u1ebft v\u1edbi nhi\u1ec1u b\u1ea3n ghi trong B\u1ea3ng B. Vi\u1ec7c tri\u1ec3n khai tr\u1ef1c ti\u1ebfp m\u1ed1i quan h\u1ec7 n\u00e0y trong c\u01a1 s\u1edf d\u1eef li\u1ec7u quan h\u1ec7 l\u00e0 kh\u00f4ng th\u1ec3 m\u00e0 kh\u00f4ng c\u00f3 m\u1ed9t c\u1ea7u n\u1ed1i.<\/p>\n

            \n
          • V\u1ea5n \u0111\u1ec1:<\/strong>B\u1ea1n kh\u00f4ng th\u1ec3 \u0111\u01a1n gi\u1ea3n th\u00eam Kh\u00f3a ngo\u1ea1i v\u00e0o m\u1ed9t b\u1ea3ng, v\u00ec m\u1ed9t h\u00e0ng s\u1ebd c\u1ea7n l\u01b0u tr\u1eef nhi\u1ec1u ID.<\/li>\n
          • Gi\u1ea3i ph\u00e1p:<\/strong> T\u1ea1o m\u1ed9t b\u1ea3ng giao nhau (sinh th\u1ec3 li\u00ean k\u1ebft).<\/li>\n<\/ul>\n

            \u0110\u1ed1i v\u1edbi Sinh vi\u00ean<\/em> v\u00e0 Kh\u00f3a h\u1ecdc<\/em>, t\u1ea1o m\u1ed9t B\u1ea3ng \u0111\u0103ng k\u00fd<\/em> b\u1ea3ng ch\u1ee9a StudentID<\/em> v\u00e0 CourseID<\/em>. \u0110i\u1ec1u n\u00e0y chuy\u1ec3n \u0111\u1ed5i m\u1ed1i quan h\u1ec7 M:N th\u00e0nh hai m\u1ed1i quan h\u1ec7 1: Nhi\u1ec1u.<\/p>\n

            7. S\u1ef1 kh\u00e1c bi\u1ec7t gi\u1eefa Cardinality v\u00e0 Modality l\u00e0 g\u00ec? \u2696\ufe0f<\/h2>\n

            Nh\u1eefng thu\u1eadt ng\u1eef n\u00e0y m\u00f4 t\u1ea3 c\u00e1c r\u00e0ng bu\u1ed9c c\u1ee7a m\u1ed9t m\u1ed1i quan h\u1ec7, th\u01b0\u1eddng b\u1ecb nh\u1ea7m l\u1eabn do k\u00fd hi\u1ec7u t\u01b0\u01a1ng t\u1ef1 nhau.<\/p>\n

              \n
            • Cardinality:<\/strong> S\u1ed1 l\u01b0\u1ee3ng t\u1ed1i \u0111a c\u00e1c th\u1ec3 hi\u1ec7n. (v\u00ed d\u1ee5: M\u1ed9t-\u0110a).<\/li>\n
            • Modality:<\/strong> S\u1ed1 l\u01b0\u1ee3ng t\u1ed1i thi\u1ec3u c\u00e1c th\u1ec3 hi\u1ec7n. (v\u00ed d\u1ee5: B\u1eaft bu\u1ed9c ho\u1eb7c T\u00f9y ch\u1ecdn).<\/li>\n<\/ul>\n

              V\u00ed d\u1ee5: M\u1ed9t Nh\u00e2n vi\u00ean<\/em> ph\u1ea3i c\u00f3 m\u1ed9t Ph\u00f2ng ban<\/em> (Modality: B\u1eaft bu\u1ed9c\/1). M\u1ed9t Ph\u00f2ng ban<\/em> c\u00f3 th\u1ec3 t\u1ed3n t\u1ea1i m\u00e0 kh\u00f4ng c\u1ea7n m\u1ed9t Nh\u00e2n vi\u00ean<\/em> (Modality: T\u00f9y ch\u1ecdn\/0).<\/p>\n

              8. M\u1ed1i quan h\u1ec7 x\u00e1c \u0111\u1ecbnh vs. M\u1ed1i quan h\u1ec7 kh\u00f4ng x\u00e1c \u0111\u1ecbnh \ud83e\udde9<\/h2>\n

              S\u1ef1 kh\u00e1c bi\u1ec7t n\u1eb1m \u1edf s\u1ef1 ph\u1ee5 thu\u1ed9c c\u1ee7a th\u1ef1c th\u1ec3 con.<\/p>\n

                \n
              • X\u00e1c \u0111\u1ecbnh:<\/strong>Entiti con kh\u00f4ng th\u1ec3 t\u1ed3n t\u1ea1i m\u00e0 kh\u00f4ng c\u00f3 entiti cha. Kh\u00f3a ngo\u1ea1i l\u00e0 m\u1ed9t ph\u1ea7n c\u1ee7a kh\u00f3a ch\u00ednh c\u1ee7a entiti con. Th\u01b0\u1eddng \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n b\u1eb1ng \u0111\u01b0\u1eddng li\u1ec1n.<\/li>\n
              • Kh\u00f4ng x\u00e1c \u0111\u1ecbnh:<\/strong>Entiti con c\u00f3 th\u1ec3 t\u1ed3n t\u1ea1i \u0111\u1ed9c l\u1eadp. Kh\u00f3a ngo\u1ea1i kh\u00f4ng ph\u1ea3i l\u00e0 m\u1ed9t ph\u1ea7n c\u1ee7a kh\u00f3a ch\u00ednh. Th\u01b0\u1eddng \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n b\u1eb1ng \u0111\u01b0\u1eddng g\u1ea1ch \u0111\u1ee9t.<\/li>\n<\/ul>\n

                Xem x\u00e9t m\u1ed9t H\u00f3a \u0111\u01a1n<\/em> (cha) v\u00e0 D\u00f2ng h\u00f3a \u0111\u01a1n<\/em> (con). D\u00f2ng h\u00f3a \u0111\u01a1n l\u00e0 x\u00e1c \u0111\u1ecbnh v\u00ec m\u1ed9t m\u1ee5c kh\u00f4ng c\u00f3 \u00fd ngh\u0129a n\u1ebfu kh\u00f4ng c\u00f3 h\u00f3a \u0111\u01a1n.<\/p>\n

                9. M\u1ed1i quan h\u1ec7 \u0111\u1ec7 quy l\u00e0 g\u00ec? \ud83d\udd04<\/h2>\n

                M\u1ed1i quan h\u1ec7 \u0111\u1ec7 quy x\u1ea3y ra khi m\u1ed9t entiti li\u00ean k\u1ebft v\u1edbi ch\u00ednh n\u00f3. \u0110i\u1ec1u n\u00e0y ph\u1ed5 bi\u1ebfn trong d\u1eef li\u1ec7u ph\u00e2n c\u1ea5p.<\/p>\n

                  \n
                • V\u00ed d\u1ee5:<\/strong> M\u1ed9t B\u1ea3ng Nh\u00e2n vi\u00ean<\/em> b\u1ea3ng n\u01a1i m\u1ed9t nh\u00e2n vi\u00ean l\u00e0 Qu\u1ea3n l\u00fd<\/em> c\u1ee7a nh\u1eefng ng\u01b0\u1eddi kh\u00e1c.<\/li>\n
                • Tri\u1ec3n khai:<\/strong> M\u1ed9t kh\u00f3a ngo\u1ea1i trong c\u00f9ng b\u1ea3ng tr\u1ecf \u0111\u1ebfn kh\u00f3a ch\u00ednh c\u1ee7a ch\u00ednh b\u1ea3ng \u0111\u00f3.<\/li>\n<\/ul>\n

                  C\u1ea5u tr\u00fac n\u00e0y h\u1ed7 tr\u1ee3 s\u01a1 \u0111\u1ed3 t\u1ed5 ch\u1ee9c ho\u1eb7c c\u00e1c danh m\u1ee5c s\u1ea3n ph\u1ea9m c\u00f3 danh m\u1ee5c con.<\/p>\n

                  10. Entiti y\u1ebfu kh\u00e1c v\u1edbi entiti m\u1ea1nh nh\u01b0 th\u1ebf n\u00e0o? \ud83c\udf31<\/h2>\n

                  M\u1ed9t Entiti m\u1ea1nh<\/strong> c\u00f3 kh\u00f3a ch\u00ednh \u0111\u1ed9c l\u1eadp v\u1edbi c\u00e1c entiti kh\u00e1c. M\u1ed9t Entiti y\u1ebfu<\/strong>kh\u00f4ng th\u1ec3 \u0111\u01b0\u1ee3c x\u00e1c \u0111\u1ecbnh duy nh\u1ea5t n\u1ebfu kh\u00f4ng c\u00f3 kh\u00f3a ch\u00ednh c\u1ee7a entiti cha.<\/p>\n

                    \n
                  • Tr\u1ef1c quan:<\/strong>C\u00e1c entiti y\u1ebfu th\u01b0\u1eddng \u0111\u01b0\u1ee3c v\u1ebd b\u1eb1ng h\u00ecnh ch\u1eef nh\u1eadt k\u00e9p.<\/li>\n
                  • Ph\u1ee5 thu\u1ed9c:<\/strong>Ch\u00fang ph\u1ee5 thu\u1ed9c v\u00e0o m\u1ed1i quan h\u1ec7 x\u00e1c \u0111\u1ecbnh.<\/li>\n<\/ul>\n

                    V\u00ed d\u1ee5: M\u1ed9t Ng\u01b0\u1eddi ph\u1ee5 thu\u1ed9c<\/em> (v\u1ee3\/ch\u1ed3ng\/con c\u00e1i) trong h\u1ec7 th\u1ed1ng c\u00f4ng ty. M\u1ed9t b\u1ea3n ghi ng\u01b0\u1eddi ph\u1ee5 thu\u1ed9c th\u01b0\u1eddng kh\u00f4ng c\u00f3 ID duy nh\u1ea5t ri\u00eang; n\u00f3 ph\u1ee5 thu\u1ed9c v\u00e0o EmployeeID<\/em> \u0111\u1ec3 \u0111\u01b0\u1ee3c x\u00e1c \u0111\u1ecbnh.<\/p>\n

                    11. Khi n\u00e0o b\u1ea1n n\u00ean s\u1eed d\u1ee5ng Kh\u00f3a T\u1ed5ng h\u1ee3p? \ud83e\udde9<\/h2>\n

                    M\u1ed9t Kh\u00f3a T\u1ed5ng h\u1ee3p<\/strong> g\u1ed3m hai ho\u1eb7c nhi\u1ec1u c\u1ed9t c\u00f9ng nhau x\u00e1c \u0111\u1ecbnh duy nh\u1ea5t m\u1ed9t h\u00e0ng. N\u00f3 \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng khi kh\u00f4ng c\u00f3 c\u1ed9t n\u00e0o \u0111\u01a1n l\u1ebb cung c\u1ea5p t\u00ednh duy nh\u1ea5t.<\/p>\n

                      \n
                    • T\u00ecnh hu\u1ed1ng:<\/strong> M\u1ed9t StudentCourse<\/em> b\u1ea3ng.<\/li>\n
                    • Kh\u00f3a:<\/strong> StudentID<\/em> + CourseID<\/em>.<\/li>\n<\/ul>\n

                      C\u1ea3 hai ID \u0111\u1ec1u kh\u00f4ng duy nh\u1ea5t ri\u00eang l\u1ebb trong ng\u1eef c\u1ea3nh n\u00e0y, nh\u01b0ng t\u1ed5 h\u1ee3p l\u1ea1i th\u00ec l\u00e0 duy nh\u1ea5t. H\u00e3y c\u1ea9n tr\u1ecdng, v\u00ec c\u00e1c kh\u00f3a t\u1ed5ng h\u1ee3p c\u00f3 th\u1ec3 l\u00e0m ph\u1ee9c t\u1ea1p m\u1ed1i quan h\u1ec7 Kh\u00f3a ngo\u1ea1i trong c\u00e1c b\u1ea3ng kh\u00e1c.<\/p>\n

                      12. Kh\u00f3a Gi\u1ea3 vs. Kh\u00f3a T\u1ef1 nhi\u00ean: Ch\u1ecdn c\u00e1i n\u00e0o? \ud83d\udd22<\/h2>\n

                      \u0110\u00e2y l\u00e0 m\u1ed9t quy\u1ebft \u0111\u1ecbnh thi\u1ebft k\u1ebf chi\u1ebfn l\u01b0\u1ee3c.<\/p>\n

                        \n
                      • Kh\u00f3a T\u1ef1 nhi\u00ean:<\/strong> M\u1ed9t thu\u1ed9c t\u00ednh th\u1ef1c t\u1ebf (v\u00ed d\u1ee5: Email, S\u1ed1 b\u1ea3o hi\u1ec3m x\u00e3 h\u1ed9i). \u01afu \u0111i\u1ec3m: C\u00f3 \u00fd ngh\u0129a. Nh\u01b0\u1ee3c \u0111i\u1ec3m: C\u00f3 th\u1ec3 thay \u0111\u1ed5i, c\u00f3 th\u1ec3 d\u00e0i, ho\u1eb7c ch\u1ee9a th\u00f4ng tin nh\u1ea1y c\u1ea3m.<\/li>\n
                      • Kh\u00f3a Gi\u1ea3:<\/strong> M\u1ed9t ID \u0111\u01b0\u1ee3c h\u1ec7 th\u1ed1ng t\u1ea1o ra (v\u00ed d\u1ee5: s\u1ed1 nguy\u00ean t\u1ef1 t\u0103ng). \u01afu \u0111i\u1ec3m: \u1ed4n \u0111\u1ecbnh, ng\u1eafn, nhanh. Nh\u01b0\u1ee3c \u0111i\u1ec3m: Kh\u00f4ng c\u00f3 \u00fd ngh\u0129a kinh doanh.<\/li>\n<\/ul>\n

                        Th\u1ef1c h\u00e0nh t\u1ed1t th\u01b0\u1eddng \u01b0u ti\u00ean Kh\u00f3a Gi\u1ea3 cho c\u1ea5u tr\u00fac b\u1ea3ng n\u1ed9i b\u1ed9, trong khi Kh\u00f3a T\u1ef1 nhi\u00ean v\u1eabn h\u1eefu \u00edch cho t\u00ecm ki\u1ebfm v\u00e0 b\u00e1o c\u00e1o.<\/p>\n

                        13. Chu\u1ea9n h\u00f3a \u1ea3nh h\u01b0\u1edfng nh\u01b0 th\u1ebf n\u00e0o \u0111\u1ebfn ERD? \ud83d\udcc9<\/h2>\n

                        Chu\u1ea9n h\u00f3a l\u00e0 qu\u00e1 tr\u00ecnh t\u1ed5 ch\u1ee9c d\u1eef li\u1ec7u \u0111\u1ec3 gi\u1ea3m thi\u1ec3u s\u1ef1 tr\u00f9ng l\u1eb7p. ERD s\u1ebd thay \u0111\u1ed5i theo qu\u00e1 tr\u00ecnh chu\u1ea9n h\u00f3a c\u1ee7a b\u1ea1n.<\/p>\n

                          \n
                        • 1NF:<\/strong> Lo\u1ea1i b\u1ecf c\u00e1c nh\u00f3m l\u1eb7p l\u1ea1i.<\/li>\n
                        • 2NF:<\/strong> Lo\u1ea1i b\u1ecf c\u00e1c ph\u1ee5 thu\u1ed9c ri\u00eang ph\u1ea7n.<\/li>\n
                        • 3NF:<\/strong> Lo\u1ea1i b\u1ecf c\u00e1c ph\u1ee5 thu\u1ed9c b\u1eafc c\u1ea7u.<\/li>\n<\/ul>\n

                          Chu\u1ea9n h\u00f3a cao th\u01b0\u1eddng l\u00e0m t\u0103ng s\u1ed1 l\u01b0\u1ee3ng b\u1ea3ng v\u00e0 m\u1ed1i quan h\u1ec7. M\u1eb7c d\u00f9 \u0111i\u1ec1u n\u00e0y c\u1ea3i thi\u1ec7n t\u00ednh to\u00e0n v\u1eb9n d\u1eef li\u1ec7u, nh\u01b0ng c\u00f3 th\u1ec3 l\u00e0m ph\u1ee9c t\u1ea1p c\u00e1c truy v\u1ea5n. C\u1ea7n c\u00e2n b\u1eb1ng m\u1ee9c \u0111\u1ed9 chu\u1ea9n h\u00f3a v\u1edbi nhu c\u1ea7u hi\u1ec7u su\u1ea5t truy v\u1ea5n.<\/p>\n

                          14. K\u00fd hi\u1ec7u Crow\u2019s Foot so v\u1edbi k\u00fd hi\u1ec7u Chen: K\u00fd hi\u1ec7u n\u00e0o l\u00e0 chu\u1ea9n? \ud83d\udc63<\/h2>\n

                          K\u00fd hi\u1ec7u \u0111\u1ec1 c\u1eadp \u0111\u1ebfn c\u00e1ch c\u00e1c m\u1ed1i quan h\u1ec7 \u0111\u01b0\u1ee3c bi\u1ec3u di\u1ec5n tr\u1ef1c quan.<\/p>\n

                            \n
                          • K\u00fd hi\u1ec7u Crow\u2019s Foot:<\/strong> S\u1eed d\u1ee5ng c\u00e1c k\u00fd hi\u1ec7u nh\u01b0 \u0111\u01b0\u1eddng th\u1eb3ng, d\u1ea5u g\u1ea1ch ch\u00e9o v\u00e0 v\u00f2ng tr\u00f2n \u1edf hai \u0111\u1ea7u \u0111\u01b0\u1eddng. R\u1ea5t ph\u1ed5 bi\u1ebfn trong c\u00e1c c\u00f4ng c\u1ee5 hi\u1ec7n \u0111\u1ea1i.<\/li>\n
                          • Chen:<\/strong> S\u1eed d\u1ee5ng h\u00ecnh thoi cho c\u00e1c m\u1ed1i quan h\u1ec7 v\u00e0 h\u00ecnh ch\u1eef nh\u1eadt cho c\u00e1c th\u1ef1c th\u1ec3. Ph\u00f9 h\u1ee3p h\u01a1n v\u1edbi h\u1ecdc thu\u1eadt.<\/li>\n<\/ul>\n

                            K\u00fd hi\u1ec7u Crow\u2019s Foot th\u01b0\u1eddng \u0111\u01b0\u1ee3c \u01b0u ti\u00ean cho tri\u1ec3n khai v\u00ec n\u00f3 ph\u1ea3n \u00e1nh tr\u1ef1c ti\u1ebfp h\u01a1n c\u00e1c r\u00e0ng bu\u1ed9c SQL. Tuy nhi\u00ean, k\u00fd hi\u1ec7u Chen r\u1ea5t t\u1ed1t cho m\u00f4 h\u00ecnh h\u00f3a kh\u00e1i ni\u1ec7m c\u1ea5p cao.<\/p>\n

                            15. S\u01a1 \u0111\u1ed3 ERD so v\u1edbi s\u01a1 \u0111\u1ed3 lu\u1ed3ng d\u1eef li\u1ec7u (DFD) \ud83d\udcca<\/h2>\n

                            Ch\u00fang ph\u1ee5c v\u1ee5 c\u00e1c m\u1ee5c \u0111\u00edch kh\u00e1c nhau trong v\u00f2ng \u0111\u1eddi thi\u1ebft k\u1ebf h\u1ec7 th\u1ed1ng.<\/p>\n

                              \n
                            • ERD:<\/strong> T\u1eadp trung v\u00e0o c\u1ea5u tr\u00fac d\u1eef li\u1ec7u<\/em> v\u00e0 l\u01b0u tr\u1eef. G\u00f3c nh\u00ecn t\u0129nh v\u1ec1 c\u00e1c m\u1ed1i quan h\u1ec7.<\/li>\n
                            • DFD:<\/strong> T\u1eadp trung v\u00e0o s\u1ef1 di chuy\u1ec3n d\u1eef li\u1ec7u<\/em> v\u00e0 c\u00e1c qu\u00e1 tr\u00ecnh. G\u00f3c nh\u00ecn \u0111\u1ed9ng v\u1ec1 c\u00e1ch d\u1eef li\u1ec7u di chuy\u1ec3n qua h\u1ec7 th\u1ed1ng.<\/li>\n<\/ul>\n

                              \u0110\u1eebng nh\u1ea7m l\u1eabn hai th\u1ee9 n\u00e0y. M\u1ed9t ERD cho b\u1ea1n bi\u1ebft d\u1eef li\u1ec7u n\u00e0o t\u1ed3n t\u1ea1i. M\u1ed9t DFD cho b\u1ea1n bi\u1ebft d\u1eef li\u1ec7u \u0111\u00f3 \u0111\u01b0\u1ee3c x\u1eed l\u00fd nh\u01b0 th\u1ebf n\u00e0o. C\u1ea3 hai \u0111\u1ec1u c\u1ea7n thi\u1ebft \u0111\u1ec3 c\u00f3 m\u1ed9t b\u1ea3n m\u00f4 t\u1ea3 h\u1ec7 th\u1ed1ng \u0111\u1ea7y \u0111\u1ee7.<\/p>\n

                              T\u00f3m t\u1eaft c\u00e1c kh\u00e1i ni\u1ec7m ch\u00ednh \ud83d\udcdd<\/h2>\n\n\n\n\n\n\n\n\n
                              Kh\u00e1i ni\u1ec7m<\/th>\nB\u00e0i h\u1ecdc ch\u00ednh<\/th>\n<\/tr>\n<\/thead>\n
                              Kh\u00f3a ch\u00ednh<\/td>\nID duy nh\u1ea5t cho m\u1ed9t h\u00e0ng. Kh\u00f4ng \u0111\u01b0\u1ee3c ph\u00e9p gi\u00e1 tr\u1ecb null.<\/td>\n<\/tr>\n
                              Kh\u00f3a ngo\u1ea1i<\/td>\nLi\u00ean k\u1ebft \u0111\u1ebfn kh\u00f3a ch\u00ednh c\u1ee7a b\u1ea3ng kh\u00e1c.<\/td>\n<\/tr>\n
                              S\u1ed1 l\u01b0\u1ee3ng<\/td>\nS\u1ed1 l\u01b0\u1ee3ng m\u1ed1i quan h\u1ec7 t\u1ed1i \u0111a (1, 1..N).<\/td>\n<\/tr>\n
                              B\u1ea3ng li\u00ean k\u1ebft<\/td>\nGi\u1ea3i quy\u1ebft c\u00e1c m\u1ed1i quan h\u1ec7 Nhi\u1ec1u-\u0110a.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

                              N\u1eafm v\u1eefng nh\u1eefng s\u1ef1 kh\u00e1c bi\u1ec7t n\u00e0y gi\u00fap thi\u1ebft k\u1ebf c\u01a1 s\u1edf d\u1eef li\u1ec7u m\u1ea1nh m\u1ebd. M\u1ee5c ti\u00eau l\u00e0 s\u1ef1 r\u00f5 r\u00e0ng, t\u00ednh to\u00e0n v\u1eb9n v\u00e0 kh\u1ea3 n\u0103ng m\u1edf r\u1ed9ng. Xem x\u00e9t l\u1ea1i s\u01a1 \u0111\u1ed3 c\u1ee7a b\u1ea1n d\u1ef1a tr\u00ean nh\u1eefng \u0111i\u1ec3m n\u00e0y \u0111\u1ec3 \u0111\u1ea3m b\u1ea3o m\u00f4 h\u00ecnh c\u1ee7a b\u1ea1n ph\u1ea3n \u00e1nh \u0111\u00fang th\u1ef1c t\u1ebf kinh doanh.<\/p>\n

                              B\u1eb1ng c\u00e1ch gi\u1ea3i quy\u1ebft 15 nh\u1ea7m l\u1eabn ph\u1ed5 bi\u1ebfn n\u00e0y, b\u1ea1n s\u1ebd x\u00e2y d\u1ef1ng n\u1ec1n t\u1ea3ng cho c\u00e1c h\u1ec7 th\u1ed1ng d\u1ec5 b\u1ea3o tr\u00ec v\u00e0 m\u1edf r\u1ed9ng. T\u1eadp trung v\u00e0o ng\u1eef ngh\u0129a d\u1eef li\u1ec7u, v\u00e0 vi\u1ec7c tri\u1ec3n khai k\u1ef9 thu\u1eadt s\u1ebd t\u1ef1 nhi\u00ean theo sau.<\/p>\n","protected":false},"excerpt":{"rendered":"

                              S\u01a1 \u0111\u1ed3 quan h\u1ec7 th\u1ef1c th\u1ec3 (ERD) \u0111\u00f3ng vai tr\u00f2 nh\u01b0 b\u1ea3n v\u1ebd thi\u1ebft k\u1ebf cho ki\u1ebfn tr\u00fac c\u01a1 s\u1edf d\u1eef li\u1ec7u. Tuy nhi\u00ean, ngay c\u1ea3 nh\u1eefng nh\u00e0 thi\u1ebft k\u1ebf…<\/p>\n","protected":false},"author":1,"featured_media":62,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_yoast_wpseo_title":"Nh\u1eefng nh\u1ea7m l\u1eabn v\u1ec1 s\u01a1 \u0111\u1ed3 ER: 15 Kh\u00f3a, M\u1ed1i quan h\u1ec7 v\u00e0 Cardinality \u0111\u01b0\u1ee3c gi\u1ea3i th\u00edch","_yoast_wpseo_metadesc":"Gi\u1ea3i quy\u1ebft c\u00e1c v\u1ea5n \u0111\u1ec1 thi\u1ebft k\u1ebf ERD ph\u1ed5 bi\u1ebfn. H\u01b0\u1edbng d\u1eabn v\u1ec1 Kh\u00f3a, Cardinality, M\u1ed1i quan h\u1ec7 v\u00e0 Chu\u1ea9n h\u00f3a. C\u00e2u h\u1ecfi v\u00e0 tr\u1ea3 l\u1eddi r\u00f5 r\u00e0ng d\u00e0nh cho c\u00e1c ki\u1ebfn tr\u00fac s\u01b0 c\u01a1 s\u1edf d\u1eef li\u1ec7u.","inline_featured_image":false,"fifu_image_url":"","fifu_image_alt":"","footnotes":""},"categories":[6],"tags":[10,11],"class_list":["post-61","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-erd","tag-academic","tag-erd"],"yoast_head":"\nNh\u1eefng nh\u1ea7m l\u1eabn v\u1ec1 s\u01a1 \u0111\u1ed3 ER: 15 Kh\u00f3a, M\u1ed1i quan h\u1ec7 v\u00e0 Cardinality \u0111\u01b0\u1ee3c gi\u1ea3i th\u00edch<\/title>\n<meta name=\"description\" content=\"Gi\u1ea3i quy\u1ebft c\u00e1c v\u1ea5n \u0111\u1ec1 thi\u1ebft k\u1ebf ERD ph\u1ed5 bi\u1ebfn. H\u01b0\u1edbng d\u1eabn v\u1ec1 Kh\u00f3a, Cardinality, M\u1ed1i quan h\u1ec7 v\u00e0 Chu\u1ea9n h\u00f3a. 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