Question

我有一组关系模式的功能依赖项，需要找到最小的封面。我理解消除冗余线路的基本概念，但我正在努力完成它。完成此任务的最有效方法是什么？

这些是功能依赖：

client      --> office

stock       --> exchange, dividend

broker      --> profile

company     --> stock

client      --> risk_profile, analyst 

analyst     --> broker

stock, broker   --> investment, volume

stock       --> company

investment  --> commission, return

stock, broker   --> client

account     --> assets

Answer 1

转换所有FD，使得任何FD的RHS仅包含单个属性

client      --> office
stock       --> exchange
stock       --> dividend
broker      --> profile
company     --> stock
client      --> risk_profile
client      -->analyst
analyst     --> broker
stock, broker   --> investment
stock, broker   --> volume
stock       --> company
investment  --> return
investment  --> commission
stock, broker   --> client
account     --> assets

我们需要寻找下一步 LHS上的冗余属性

选择在LHS上具有2个或2个以上属性的FD

1.stock, broker   --> investment

一次从LHS中删除一个属性，如果属性的clouser包含已删除的属性，则计算剩余属性的clouser，然后您可以实际删除该属性。

删除股票表格1并计算经纪人的clouser

(broker)+ = {broker,profile,investment,return ,commission}

不包含库存，因此您无法删除库存

删除经纪人表单1并计算库存的clouser

(stock)+ = {stock,exchange,dividend,investment,return,commission,company}

不包含经纪人，因此您无法删除经纪人

你可以玩跟随FD的同一游戏

2.stock, broker   --> volume
3.stock, broker   --> client

对于FD 3.你会发现可以删除经纪人，导致跟随FD

client      --> office
stock       --> exchange
stock       --> dividend
broker      --> profile
company     --> stock
client      --> risk_profile
client      -->analyst
analyst     --> broker
stock, broker   --> investment
stock, broker   --> volume
stock       --> company
investment  --> return
investment  --> commission
stock       --> client
account     --> assets

最后一步是寻找多余的FD。检查X形式的FD ---＆gt; Y是X的冗余计算缓冲区并检查它是否包含Y，如果是这种情况，则可以安全地从最小覆盖集中删除FD。如下所示。

 client      --> office

计算客户端的clouser

(client)+ = { client , risk_proflie,analyst,broker,profile }

clouser不包含办公室，因此您无法将其删除。重复最后一步，你会发现没有FD可以删除，因此最小的封面集是

client      --> office
stock       --> exchange
stock       --> dividend
broker      --> profile
company     --> stock
client      --> risk_profile
client      -->analyst
analyst     --> broker
stock, broker   --> investment
stock, broker   --> volume
stock       --> company
investment  --> return
investment  --> commission
stock       --> client
account     --> assets

找到给定的一组函数依赖项的最小覆盖

1 个答案: