Existential quantification - sql, PL-SQL Programming

Assignment Help:

Existential Quantification - SQL

Existential quantification-stating that something is true of at least one object under consideration-can be expressed by OR(r,c), meaning that at least one object that satisfies a predicate for r also satisfies c, and IS_NOT_EMPTY(r). The names for the aggregate operators AND and OR reflect the facts that when we confine our attention to finite sets, universal and existential quantification are equivalent to repeated invocations of dyadic AND and dyadic OR, respectively. Note that AND(r,c) is equivalent to COUNT(r) = COUNT(r WHERE c), and OR(r,c) is equivalent to COUNT(r WHERE c) > 0 and also to IS_NOT_EMPTY(r WHERE c).

Quantification also appears in various guises in SQL, but its meaning is muddied by those same two violations of relational theory that we have already seen muddying the waters: duplicate rows and NULL. For example, SQL's (SELECT COUNT(*) FROM r), a so-called scalar subquery (because it is an expression denoting a table with one row and one column, enclosed in parentheses), denotes the number of rows in the table r, but can we really say that this represents the number of objects that satisfy a predicate for r, if the same row can be counted more than once, or if NULL appears in place of a column value in some row of r? In fact, what might it mean to say that a row does or does not satisfy a predicate? In 2VL we say that object a satisfies predicate P(x) exactly when P(a) is true. Does this still hold in 3VL, or might SQL deem a to satisfy P(x) also when P(a) is unknown? Well, it turns out that SQL uses both interpretations, depending on the context, as we shall discover.

 


Related Discussions:- Existential quantification - sql

Using forall statement - bulk bind performance improvement, Using the FORAL...

Using the FORALL Statement The keyword FORALL instruct the PL/SQL engine to bulk-bind input collections before sending them all to the SQL engine. Though the FORALL statement

Nested tables versus index-by tables, Nested Tables versus Index-by Tables ...

Nested Tables versus Index-by Tables The Index-by tables and nested tables are just similar. For e.g.  They have similar structure and their individual elements are accessed in

Lob types in pl/sql, LOB Types The large object (LOB) datatypes like BF...

LOB Types The large object (LOB) datatypes like BFILE, BLOB, CLOB, and NCLOB store the blocks of unstructured data (like graphic images, text, video clips, and sound waveforms)

Processing transactions, Processing Transactions This part describes ho...

Processing Transactions This part describes how to do the transaction processing. You learn the fundamental techniques that safeguard the consistency of your database, involvin

Prepare a slq project, Project Description: I want to write some SQL st...

Project Description: I want to write some SQL statements. The things I need are between pages 5-7. The only problem is that i want it till tomorrow. Skills required is SQL

Manipulating local collections - pl/sql, Manipulating Local Collections ...

Manipulating Local Collections Within PL/SQL, to manipulate the local collection, by using the  TABLE and CAST operators . The operands of CAST are a collection declared locally

Cursor attributes in pl sql, Cursor Attributes   The Cursors and curso...

Cursor Attributes   The Cursors and cursor variables have 4 attributes which give you helpful information about the execution of a data manipulation statement. Syntax:

Use serially reusable packages - performance of application, Use Serially R...

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

Product-specific packages in pl/sql, Product-specific Packages The Ora...

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 illu

Boolean values-assignments in pl/sql, Boolean Values Only the values TRU...

Boolean Values Only the values TRUE, FALSE, & NULL can be assigned to a Boolean variable. For illustration, given the declaration DECLARE done BOOLEAN; the following statements

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd