请问大侠： superkey - 未名空间MITBBS历史存档

国际科技财经博客移民网络热点娱乐民生时事公众号

Redian新闻

>未名空间

>Database - 数据库

请问大侠： superkey

请问大侠： superkey# Database - 数据库

w*g2001-04-25 07:04

1 楼

suppose that R is given with attributes ( A,B,C,D,E,F) and
the FDs ( A-->BC, B-->E, CD-->EF)
i would like to know, for the superkey at here. why not can
just AD be the superkey. as i found the superkey's
defination : K is a superkey of a relation R iff K+ consists
of all attributes of R, i think ( A, D)+ = ( A,B,C,D, E ,F).
thus why not only the AD can be the superkey and has to plus
B and C.
thank you very much for your help. good day

m*n2001-04-25 07:04

2 楼

AD is the key for the relation. Key is the minimum set of attributes
that FD determines the relation, while superkey is not the minimum set.
so ADC, ADB, ADE, are all superkeys.

【在 w******g 的大作中提到】

: suppose that R is given with attributes ( A,B,C,D,E,F) and
: the FDs ( A-->BC, B-->E, CD-->EF)
: i would like to know, for the superkey at here. why not can
: just AD be the superkey. as i found the superkey's
: defination : K is a superkey of a relation R iff K+ consists
: of all attributes of R, i think ( A, D)+ = ( A,B,C,D, E ,F).
: thus why not only the AD can be the superkey and has to plus
: B and C.
: thank you very much for your help. good day