标准化：什么是“重复基团”意思？ [英] Normalization: What does "repeating groups" mean?
问题描述
我已经阅读了不同的教程，并看到了不同的标准化例子，特别是第一个正常形式的重复组的概念。从他们我已经收集，重复组是种类多值属性（例如设计原则。因为它通常被认为是差的设计，它适合数据库设计者，有时甚至教师将这种重复的列称为重复组和违反第一正常形式的精神。
这种对术语的非正式使用是有点不幸的，因为它可能有点武断和混乱（一组列实际上构成一个重复？），也是因为它是一个更基本的问题的分心，即Null问题。所有正常形式都涉及不允许无效的可能性的关系。如果表允许任何列中的空值，那么它不满足满足1NF的关系模式的要求。在我们的Families表的情况下，如果Child列允许null（代表少于N个孩子的家庭），则Families表不满足1NF。在标准化练习中，null的可能性常常被遗忘或忽略，但是避免不必要的可空列是避免重复的列集合的一个非常好的理由，无论您是否将它们称为重复组。
另请参见本文。
I have read different tutorials and seen different examples of normalization, specially the notion of "repeating groups" in the first normal form. From them I have have gathered that repeating groups are "kind-of" multi-valued attributes (e.g. here and here).
But we already make separate tables for each multi-valued attribute by including foreign keys from the parent table during the process of mapping an ERM (Entity relationship Model) to a RDM (Relational Data Model)? Reference: this
Secondly, are those "repeating groups" essentially laid out horizontally in the same row, or can the same value occurring in the same column again and again, i.e. the same value of an attribute again and again, also a repeating group and should be eliminated?
In this example, the value English is repeating again and again. Is this a repeating group? If I eliminate it to make another table SUBJECT with Subject Name and Module_ID(Foreign key), this is what I get. Sure it gets rid of the repeating value, but I am not sure if this is the right thing. Is it right?
The term "repeating group" originally meant the concept in CODASYL and COBOL based languages where a single field could contain an array of repeating values. When E.F.Codd described his First Normal Form that was what he meant by a repeating group. The concept does not exist in any modern relational or SQL-based DBMS.
The term "repeating group" has also come to be used informally and imprecisely by database designers to mean a repeating set of columns, meaning a collection of columns containing similar kinds of values in a table. This is different to its original meaning in relation to 1NF. For instance in the case of a table called Families with columns named Parent1, Parent2, Child1, Child2, Child3, ... etc the collection of Child N columns is sometimes referred to as a repeating group and assumed to be in violation of 1NF even though it is not a repeating group in the sense that Codd intended.
This latter sense of a so-called repeating group is not technically a violation of 1NF if each attribute is only single-valued. The attributes themselves do not contain repeating values and therefore there is no violation of 1NF for that reason. Such a design is often considered an anti-pattern however because it constrains the table to a predetermined fixed number of values (maximum N children in a family) and because it forces queries and other business logic to be repeated for each of the columns. In other words it violates the "DRY" principle of design. Because it is generally considered poor design it suits database designers and sometimes even teachers to refer to repeated columns of this kind as a "repeating group" and a violation of the spirit of the First Normal Form.
This informal usage of terminology is slightly unfortunate because it can be a little arbitrary and confusing (when does a set of columns actually constitute a repetition?) and also because it is a distraction from a more fundamental issue, namely the Null problem. All of the Normal Forms are concerned with relations that don't permit the possibility of nulls. If a table permits a null in any column then it doesn't meet the requirements of a relation schema satisfying 1NF. In the case of our Families table, if the Child columns permit nulls (to represent families who have fewer than N children) then the Families table doesn't satisfy 1NF. The possibility of nulls is often forgotten or ignored in normalization exercises but the avoidance of unnecessary nullable columns is one very good reason for avoiding repeating sets of columns, whether or not you call them "repeating groups".
See also this article.
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