m******g
发帖数: 91 | 1 '选修'了一们database, 可是脑瓜不济当年啦。。。好多问题不懂的说。
向大虾们请教3个问题先：
1) Why is BCNF a good relational database design objective?
不懂，因为BCNF is said to be optimum ONLY in principle, whilst 3NF is more preferable to
database designer.. :P
2) Why is the relational data model currently more successful than the hierachical data mode?
只是因为他使用primary key 和referential key么？
3) 怎样证明：
if R is in 3NF, and there is a unique key for R (i.e. R has only
one key w.r.t F) 则R is in BCNF?
10G的thanks.. :)))) | h***y
发帖数: 3 | 2
(R is 3NF) and (R have only one key) => if there is f.d.
X->A, A can not be prime attribute(otherwise there will be
two keys). =>X must be the key => it's BCNF
【在 m******g 的大作中提到】
: '选修'了一们database, 可是脑瓜不济当年啦。。。好多问题不懂的说。
: 向大虾们请教3个问题先：
: 1) Why is BCNF a good relational database design objective?
: 不懂，因为BCNF is said to be optimum ONLY in principle, whilst 3NF is more preferable to
: database designer.. :P
: 2) Why is the relational data model currently more successful than the hierachical data mode?
: 只是因为他使用primary key 和referential key么？
: 3) 怎样证明：
: if R is in 3NF, and there is a unique key for R (i.e. R has only
: one key w.r.t F) 则R is in BCNF?
| a*****e
发帖数: 1700 | 3
NF 高其实不是件好事，因为 to query data scattered in many tables, you
need multiple join. 而 JOIN 比普通 SELECT 要 expensive 多得多，少用为
好。
主要是 efficiency. hierachical data model 没有一个很好的方式来提高
查询和更新的效率，其实并非不可能，只是和 relational database 还差
了老大一截。而且较多问题用 relational 的方式就可以比较方便的 model
出来，而且实际程序简单还快。
很明显的一个例子就是 SQL 这么傻的 language 得到这么广的应用，而
hierachical db query language 到现在都没有个好的得到公认的方案。
(别跟我说 LDAP，那还不够格 :) 我用过 CPL，个人认为 it beats the
shit out of SQL，但是恐怕没有什么人知道。
其实 mathematically beautiful 的东西多了，不过因为无人理解，或者
无人 bother to und
【在 m******g 的大作中提到】
: '选修'了一们database, 可是脑瓜不济当年啦。。。好多问题不懂的说。
: 向大虾们请教3个问题先：
: 1) Why is BCNF a good relational database design objective?
: 不懂，因为BCNF is said to be optimum ONLY in principle, whilst 3NF is more preferable to
: database designer.. :P
: 2) Why is the relational data model currently more successful than the hierachical data mode?
: 只是因为他使用primary key 和referential key么？
: 3) 怎样证明：
: if R is in 3NF, and there is a unique key for R (i.e. R has only
: one key w.r.t F) 则R is in BCNF?
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