标准化:什么是“重复组"?意思是? [英] Normalization: What does "repeating groups" mean?
问题描述
我阅读了不同的教程，看到了不同的规范化示例，特别是第一范式中重复组"的概念.从他们那里我收集到重复组是某种"多值属性(例如 "设计原则.因为它通常被认为是糟糕的设计，所以适合数据库设计人员，有时甚至教师将这种重复列称为重复组"，违反了第一范式的精神.
这种非正式的术语用法有点令人遗憾，因为它可能有点武断和混乱(一组列实际上何时构成重复?)而且因为它分散了对更基本问题的注意力，即空问题.所有范式都关注不允许空值的关系.如果表允许任何列中的空值，则它不满足满足 1NF 的关系模式的要求.对于我们的 Families 表，如果 Child 列允许空值(代表少于 N 个孩子的家庭)，则 Families 表不满足 1NF.在规范化练习中经常会忘记或忽略空值的可能性，但避免不必要的可为空列是避免重复列集的一个很好的理由，无论您是否称它们为重复组".
另见这篇文章.
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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