Already have an account? Get multiple benefits of using own account!
Login in your account..!
Remember me
Don't have an account? Create your account in less than a minutes,
Forgot password? how can I recover my password now!
Enter right registered email to receive password!
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.
Example of Null operator - NiNo Rule If we wanted to make HIGHER_OF adhere to "NULL in, NULL out"-let's call it the NiNo rule-we would have to write something like what is sho
Advantages of Subprograms The Subprograms give extensibility; that is, tailor the PL/SQL language to suit your requirements. For illustration, if you require a procedure which
Initializing and Referencing Collections Until you initialize a collection, a nested table or varray is automatically null (i.e. the collection itself is null, not its elements)
Using PRIOR and NEXT The PRIOR(n) returns the index number that precede index n in a collection. The NEXT(n) returns the index number which succeed the index n. If n has no pr
Extension and AND in SQL The theory book gives the following simple example of relational extension in Tutorial D: EXTEND IS_CALLED ADD ( FirstLetter ( Name ) AS Initial )
Multiset types - SQL An SQL multiset is what in mathematics is also known as a bag-something like a set except that the same element can appear more than once. The body of an
Complete the following steps to create a procedure to calculate the tax on an order. The BB_TAX table contains the states that require taxes to be submitted for Internet sales. If
Project Description: I want a small relational database to be built. I want the database to have the subsequent information tables: Employee Information Document storage
Mixed Notation The fourth procedure call shows that you can mix the positional and named notation. In this situation, the first parameter uses the positional notation, & the s
Example of ADD CONSTRAINT in SQL Example: Alternative formulation for MAX_ENROLMENTS ALTER TABLE IS_ENROLLED_ON ADD CONSTRAINT MAX_ENROLMENTS CHECK ((SELECT COUNT (*)
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!
whatsapp: +1-415-670-9521
Phone: +1-415-670-9521
Email: [email protected]
All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd