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11招教你如何玩转数据库设计 (11 Important Database designing rules)(七)
2012-04-13 14:36:15 来源:c-sharpcorner 作者:Shivprasad 【 】 浏览:43917次 评论:0

Rule 7:- Choose derived columns preciously

a11.jpg

If you are working on OLTP applications must be getting rid of derive columns would be good thought, until there is some pressing reason of performance. In case of OLAP where we do lot of summations, calculations these kinds of fields are necessary to gain performance.

In the above figure you can see how average field is dependent on marks and subject. This is also one of form of redundancy. So for such kind of fields which are derived from other fields give a thought are they really necessary.

This rule is also termed as 3rd normal form "No columns should depend on other non-primary key columns". My personal thought is do not apply this rule blindly see the situation; it's not that redundant data is always bad. If the redundant data is calculative data , see the situation and then decide do you want to implement the third normal form.
 

Rule 7:选择派生列

如果你想进行OLTP应用首先得筛选出派生列,在OLAP中我们需要做一些求和,方可获得uixie很好的性能。如图,求的平均数需要利用marks和subject两列。

这个规则被称为第三范式,“不应该有依赖于非主键的列”(No columns should depend on other non-primary key columns)我个人认为是不能盲目使用此规则。如果该数据是计算过的数据,看清状况然后在决定实施第三范式。

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