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 %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

Using extend - collection method, Using EXTEND To enlarge the size of ...

Using EXTEND To enlarge the size of a collection, use EXTEND. This process has 3 forms. The EXTEND appends one null element to a collection. And the EXTEND(n) appends n null e

Parameter and keyword description - exit statement, Parameter and Keyword D...

Parameter and Keyword Description: EXIT: An unconditional EXIT statement (i.e., one without a WHEN clause) exits the present loop instantly. The Execution resumes with th

Biconditional - sql, Biconditional - SQL The biconditional p ↔ q can b...

Biconditional - SQL The biconditional p ↔ q can be expressed in Tutorial D by p = q and the same is true of SQL. The question then arises as to whether, in SQL, p = q is equiv

Overriding default locking, Overriding Default Locking By default, the...

Overriding Default Locking By default, the Oracle locks the data structures for you automatically. Though, you can request exact data locks on rows or tables when it is to you

Example of group by and collect operator, Example of GROUP BY and COLLECT O...

Example of GROUP BY and COLLECT Operator Example: Using GROUP BY and COLLECT to obtain C_ER2 SELECT CourseId, CAST ( COLLECT (ROW (StudentId, Mark)) AS ROW (Studen

Table comparison - sql, Table Comparison - SQL The following definitio...

Table Comparison - SQL The following definitions for relation comparisons: Let r1 and r2 be relations having the same heading. Then: r1 ⊆ r2 is true if every tuple of r1

Parameter and keyword description - insert statement, Parameter and Keyword...

Parameter and Keyword Description:   table_reference: This identifies a table or view which should be available when you execute the INSERT statement, and for that you sho

Opening a cursor, Opening a Cursor Opening the cursor executes the que...

Opening a Cursor Opening the cursor executes the query & identifies the result set that consists of all rows that meet the query search criteria. For the cursors declared usin

Primary key, PRIMARY KEY: PRIMARY KEY  indicates that the table is sub...

PRIMARY KEY: PRIMARY KEY  indicates that the table is subject to a key constraint, in this case declaring that no two rows in the table assigned to ENROLMENT can ever have the

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