Generalized joins
Generalized joins
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广义连接
DOI：
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发表时间：
1976
期刊：
影响因子：
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通讯作者：
A. Pirotte
中科院分区：
文献类型：
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作者：
M. Lacroix;A. Pirotte
在[I]中，Codd 提出了一种三值逻辑来描述关系代数的常用运算中空值的行为。这篇简短的注释介绍了连接操作的概括，这些操作的定义方式使得连接中不会丢失任何信息，即可以从连接中恢复操作数关系。所提出的操作在两个不包含空值的关系的连接中引入空值（用“~”表示）。如果它们确实包含 m，那么可以通过应用[11.]中给出的规则轻松修改连接。下面给出 R(A, BI) 与 S(B2, C) 的联接的广义联接的定义，其中 BI 和 B2 参与联接。修改关系域的数量只是一个符号问题。以下扩展将被视为示例： R S R [B1 ~ B2 ]S A B1 B2 CA B1 B2 Sum n a up p p b u ~ p b w n n a w n q c x p p b x p u m m x r x r m ~ q c 上面的示例显示了广义等值连接 R [ BI ~ B2 ]S。 U {~,~>} ® (s*(s[1]-R[2]))。
In [I ], Codd proposes a three-valued logic to describe the behavior of null values in the usual operations of a relational algebra. This short note presents generalizations of the join operation which are defined in such a manner that no information is lost in the join, i.e. that the operand relations can be recovered from the join. The proposed operations introduce null values (denoted by "~") in the join of two relations which are supposed to contain no null value. If they do contain m, then the joins are easily modified by applying the rules given in [11. The definition of the generalized joins are given below for the join of R(A, BI) with S(B2, C), where BI and B2 participate in the join. Modifying the number of domains of the relations is simply a matter of notations. The following extensions will be considered as an example : R S R [B1 ~ B2 ]S A B1 B2 C A B1 B2 S u m n a u p p b u ~ p b w n n a w n q c x p p b x p u m m x r x r m ~ q c The generalized equi-join R [ BI ~ B2 ]S is shown above for the example. U {~,~>} ® (s*(s [ 1 ]-R [2 ])).