Boyce-Codd Oddiy formasi (BCNF) nima?

by Mike Chapple

BCNF uzilishlarni kamaytiradi va ma'lumotlar yaxlitligini oshiradi

Boyce-Codd Oddiy shaklining maqsadi (BCNF) ma'lumotlar bazasini normallashtirishga erishish uchun relaksiya bazasining ustunlari va jadvallarini tashkil qilish orqali ma'lumotlar yaxlitligini oshirishdan iborat. Ma'lumotlar bazasi normallashishi jadvallar orasidagi munosabatlar o'rnatilganda va jadvallar ma'lumotlar bazasini yanada moslashuvi va ma'lumotlarni saqlab qolish uchun qoidalar belgilagan holatlarda yuzaga keladi.

Ma'lumotlar bazalarini me'yoriylashtirish maqsadlari kerak bo'lmagan ma'lumotlarni yo'qotish va ma'lumotlar uzgaruvchilarining mantiqiyligini ta'minlashdir.

Agar ma'lumotlar bazasi bir nechta jadvalda saqlanmagan bo'lsa va faqat tegishli ma'lumot jadvalda saqlansa, ma'lumotlar bazasi normallashadi.

Boyce-Codd Normal Formining kelib chiqishi

Bir qator ko'rsatmalarga muvofiq, ma'lumotlar bazalari normallashtirilishini ta'minlang. Ushbu ko'rsatmalar an'anaviy shakllar deb nomlanadi va birdan beshgacha raqamlanadi. Birlashtiruvchi ma'lumotlar bazasi birinchi uch shaklga to'g'ri kelib qolsa, normallashtiriladi: 1NF, 2NF va 3NF.

BCNF 1974 yilda Raymond Boyce va Edgar Codd tomonidan uchinchi an'anaviy shaklga yoki 3NFga kengaytma sifatida yaratilgan. Erkaklar hisoblash vaqtini qisqartirish maqsadida qo'shimcha ishlarni minimal darajada kamaytiradigan ma'lumotlar bazasi sxemalarini yaratish ustida ish olib borishdi. Uchinchi an'anaviy shakl birinchi va ikkinchi an'anaviy shakllarda ko'rsatmalarga javob beradigan qo'shimcha ravishda asosiy kalitga bog'liq bo'lmagan ustunlarni olib tashlaydi. Ba'zida 3.5NF deb ataladigan BCNF, 3NFning barcha talablariga javob beradi va nomzod kalitlari jadvaldagi boshqa atributlarga bog'liq emas.

BCNF tashkil etilguniga qadar, Boyce Coddning relatsion modeli yordamida ma'lumotni takomillashtirishga yordam beradigan Structured English Query Language (SQL) ning asosiy ishlab chiquvchilaridan biri bo'lgan. Ushbu modelda Codd ma'lumotlar bazalarining tizimli kompleksligini qisqartirishni talab qildi, bu so'rovlar yanada kuchli va moslashuvchan bo'lishi mumkin degan ma'noni anglatadi.

O'zining ma'lumot bazasi tushunchalarini ishlatib Codd 1NF, 2NF va 3NF ko'rsatmalarini aniqladi. BCNFni belgilash uchun Boys bilan birlashdi.

Nomzod kalitlari va BCNF

Nomzod kaliti - ma'lumotlar bazasida noyob kalitni tuzadigan jadval ustunlari yoki ustunlar birikmasi. Atributlarning kombinatsiyasi boshqa ma'lumotlar bilan bog'lanmasdan ma'lumotlar bazasi yozuvini aniqlash uchun ishlatilishi mumkin. Har bir jadval bir nechta nomzod kalitlarini o'z ichiga olishi mumkin, ulardan har biri asosiy kalit sifatida tan olinishi mumkin. Jadvalda faqat bitta asosiy kalit mavjud.

Nomzod kalitlari noyob bo'lishi kerak.

Agar har bir determinant nomzodning kaliti bo'lsa, u holda BCNF munosabatlar mavjud. Ishchi ma'lumotlarini saqlaydigan va , , va xususiyatlariga ega bo'lgan ma'lumotlar bazasi jadvalini ko'rib chiqing. </p> <p> Ushbu jadvalda <Employee_Id> maydonida first_name va last_name belgilanadi. 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