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

Truth tables , Truth Tables: However in propositional logic - here we ...

Truth Tables: However in propositional logic - here we are restricted to expressing sentences and where the propositions are true or false - so we can check where a particular

Close statement in pl sql, CLOSE Statement The CLOSE statement allows ...

CLOSE Statement The CLOSE statement allows the resources held by a cursor variable or open cursor to be reused. No more rows can be fetched from the cursor variable or closed

Set operators - sql operators, Set Operators The Set operators combine...

Set Operators The Set operators combine the results of the two queries into one result. The INTERSECT returns all the distinct rows selected by both queries. The MINUS returns

Assignment statement in pl sql, Assignment Statement: The assignment s...

Assignment Statement: The assignment statement sets the present value of the variable, parameter, field, or element. The statement consists of an assignment target followed by

Operator precedence-pl/sql expressions , Operator Precedence The operati...

Operator Precedence The operations within an expression are completed in a particular order depending on their precedence (priority). The table shows the default order of the op

I need sql - wp sql expert, I need SQL , WP SQL Expert Project Descripti...

I need SQL , WP SQL Expert Project Description: Expert required to modify WP SQL query. Skills required are MySQL, SQL, PHP, Wordpress

Change sql file into csv for product registration, Change Sql file into CSV...

Change Sql file into CSV for product registration on Magento Project Description: I have a set of files that are in Sql format and could like for a developer to help me with

CURSOR, #quesWrite a cursor to open an employee database and fetch the empl...

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

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

Dbms, DBMS: The answer to this question is of course given in of the t...

DBMS: The answer to this question is of course given in of the theory book. This book is concerned with SQL DBMSs and SQL databases in particular. Soon we will be looking a

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