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
Query: SELECT * FROM EMPLOYEE1; Select 5 columns and all rows from one table Query: SELECT C_ID, COMPANY, BUILDING, DEPARTMENT, BRANCH FROM CONTRACT;

Defining REF CURSOR Types To make cursor variables, you take 2 steps. At first, you define a REF CURSOR type, and then declare the cursor variables of that type. You can defin

Use Serially Reusable Packages To help you to manage the use of memory, the PL/SQL gives the pragma SERIALLY_ REUSABLE that mark some packages as serially reusable . So mark

Error Handling The PL/SQL makes it easy to detect and process the predefined and user-defined error conditions known as exceptions. Whenever an error occurs, an exception is ra

#quesWrite a cursor to open an employee database and fetch the employee record whose age is greater than 45.tion..

Product-specific Packages The Oracle and different Oracle tools are supplied with the product-specific packages which help you to build the PL/SQL-based applications. For illus

Passing Cursor Parameters You use the OPEN statement to pass the parameters to a cursor. Unless you want to accept the default values, each proper parameter in the cursor decl

Interesting properties of CROSS JOIN - SQL Compare these with the "interesting properties of JOIN", CROSS JOIN is associative but not commutative. Unlike JOIN and NATURAL JOI

Using Operator VALUE: As you may expect, the operator VALUE returns the value of an object. The VALUE takes its argument a correlation variable.  For illustration, to return a

Need Database Development with Analysis Tools Project Description: I want a database for large governmental and private data sets on one country that can be simply extended t