Table represents an extension - sql, PL-SQL Programming

Table Represents an Extension - SQL

It describes how each tuple in a relation represents a true instantiation of some predicate and each true instantiation is represented by some tuple in that relation. Thus, a relation represents an extension, its body containing exactly one tuple corresponding to each element of the extension. It is true that some SQL tables can be interpreted in this way but it is also true that some SQL tables cannot. In fact there are at least two distinct ways in which an SQL table cannot be thus interpreted:

a) In SQL it is possible for the same row to appear more than once in a table. Moreover, if tables t1 and t2 differ only in the number of appearances of some row, then that difference is significant-they are not the same table.

b) Although I have noted that in SQL the instantiation 5 < NULL cannot be considered to appear in either the extension of a < b or NOT (a < b), the row (5, NULL) can appear in a table. What could be the corresponding predicate? It would have to be some dyadic predicate, P (a, b) say, such that P (5, NULL) is true. But if NULL stands for "some value but we don't know which", how could that row appear in the same table as, say, (6, 12)? If (6, 12) means "6 is related to 12" then (5, NULL), in relational theory, would have to mean that 5 is related to NULL in that same way. But it can't, because NULL doesn't designate anything. If instead it means "5 is related to something whose identity is unknown", then we have a sentence in which nothing appears in substitution for the parameter b.

Posted Date: 1/18/2013 3:09:33 AM | Location : United States







Related Discussions:- Table represents an extension - sql, Assignment Help, Ask Question on Table represents an extension - sql, Get Answer, Expert's Help, Table represents an extension - sql Discussions

Write discussion on Table represents an extension - sql
Your posts are moderated
Related Questions
TYPES OF EVOLUTION - Sequential evolution                  :                    Minor changes in the gene pool of a population from one generation to the next, with the resul

Explicit Cursors The set of rows returned by the query can include zero, one, or multiple rows, depending on how many rows meet your search criteria. Whenever a query returns

IS NULL Operator The IS NULL operator returns the Boolean value TRUE whenever its operand is null or FALSE if it is not null. The comparisons including the nulls always yield NU

EXCEPTION_INIT Pragma The pragma EXCEPTION_INIT relates an exception name with an Oracle error number. Which allow you to refer to any internal exception by the name and to wri

Semidifference via EXCEPT and JOIN - SQL SELECT * FROM (SELECT StudentId FROM IS_CALLED WHERE Name = 'Devinder' EXCEPT DISTINCT CORRESPONDING SELECT StudentId

set serveroutput on declare a number(5); b number(5); c number(5); begin a:=&a; b:=&b; c:=a/b; dbms_output.put_line(c); exception when zero_divide then d

Functions The function is a subprogram which can take parameters and be invoked. Normally, you can use a function to calculate a value. The function has 2 sections: the specifi

Demonstrate your knowledge of PL/SQL programming by writing and thoroughly testing triggers and stored procedures associated with an e-commerce application that provides security l

Using FOR UPDATE If you declare a cursor which will be referenced in the CURRENT OF clause of an UPDATE or DELETE statement, you should use the FOR UPDATE clause to obtain an