45523

Разложение без потерь. Теорема. Примеры

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Информатика, кибернетика и программирование

Договоримся, что малыми латинскими буквами мы будем обозначать таблицы, большими латинскими буквами – атрибуты и множества атрибутов. Например, r(R) – это таблица r со множеством атрибутов R

Русский

2013-11-17

29.5 KB

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Билет №3

Разложение без потерь. Теорема. Примеры.

Реляционная алгебра.

Договоримся, что малыми латинскими буквами мы будем обозначать таблицы, большими латинскими буквами – атрибуты и множества атрибутов. Например, r(R) – это таблица r со множеством атрибутов R.

R(A,B,C.D) – ключевые атрибуты подчеркиваются  - R={A,B,C,D}.

Условие разложения без потерь.

q(R,S)     r=    s=

 r       s=     =q’

q’q(R,S) т.е.  q’ есть надмножество q

Если  q’=q, то это разложение без потерь.

Пример:

R(X,Y,Z)             r1(X,Y)         r2(X,Z)                r1       r2 = (X,Y,Z)

      123                      12                  13                                        123

      323                      32                  33                                        127

      117                      11                  17                                        122

      132                      13                  12                                        323

      417                      41                  47                                        113

117

112

133

137

132

417

то есть это разложение с потерями.

Рассмотрим другое измерение:

 r3(X,Y)             r4(Y,Z)                   r3      r4 = (X,Y,Z)

       12                    23                                          123

       32                    17                                          323

       11                    32                                          117

       13                                                                  132

  1.   417

это разложение без потерь.


 

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