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

Cursor attributes in dynamic sql - pl sql, Using Cursor Attributes: Ev...

Using Cursor Attributes: Every cursor has 4 attributes: %NOTFOUND, %FOUND, %ISOPEN, and %ROWCOUNT. If appended to the cursor name, they return the helpful information about

Control structure, Control Structures The Control structures are the mo...

Control Structures The Control structures are the most important PL/SQL extension to the SQL. Not only does PL/SQL let you manipulate Oracle data, it lets you process the data

Opening a cursor variable, Opening a Cursor Variable The OPEN-FOR stat...

Opening a Cursor Variable The OPEN-FOR statement relates a cursor variable with the multi-row query, executes the query, and then identifies the result set. The syntax for ope

Types of evolution, TYPES OF EVOLUTION - Sequential evolution         ...

TYPES OF EVOLUTION - Sequential evolution                  :                    Minor changes in the gene pool of a population from one generation to the next, with the resul

Organic evolution, Organi c Evolution Evolution is a slow continuou...

Organi c Evolution Evolution is a slow continuous, irreversible and natural process of change to give rise to advance and diverse forms of life i.e. formation of new specie

Keys in sql, Keys in SQL SQL support for keys in the following respect...

Keys in SQL SQL support for keys in the following respects: SQL does not require at least one key for every base table. If no key is explicitly declared, then KEY {ALL B

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

Named notation, Named Notation The second procedure call uses the name...

Named Notation The second procedure call uses the named notation. An arrow (=>) serve as the relationship operator that associates the formal parameter to the left of the arro

Using %rowtype-declarations in sql, Using %ROWTYPE The %ROWTYPE attribut...

Using %ROWTYPE The %ROWTYPE attribute gives a record type which represents a row in a table (or view). The record can store the whole row of data selected from the table or fetc

%rowcount, %ROWCOUNT When its cursor or cursor variable is opened, the...

%ROWCOUNT When its cursor or cursor variable is opened, the %ROWCOUNT is zeroed. Before the first fetch, the %ROWCOUNT yields 0. Afterward, it yields the number of rows fetche

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