Conditionals - sql, PL-SQL Programming

Assignment Help:

Conditionals - SQL

At first sight SQL does not appear to have a single operator for expressing logical implication. In this respect it would be in common with most programming languages, including Tutorial D. However, standard SQL defines a partial ordering for its three truth values, under which false is deemed to precede true. Thus, the comparisons p < q, p > q, p <= q, and p >= q are all supported in standard SQL (in addition to p = q, of course).

it is noted that in 2VL p → q is equivalent to ¬ p ∨ q. Study of Figure 3.5 reveals that ¬ p ∨ q does indeed equate to p → q when neither operand is unknown, and the same is true of p <= q! (It is the pronunciation, "is less than or equal to", rather than "implies", that led to my observation that SQL appears to include direct support for a 3VL form of implication by accident.)

1570_Conditionals - SQL.png

Figure: The SQL Truth Tables for ¬ p∨ q and p <= q

Note, however, that p <= q is not equivalent to ¬ p ∨ q. Intuitively, we understand that "p implies q" is true whenever q is true. This holds for ¬ p ∨ q but not for p <= q, as the row for p = U and q = T shows. The U in the last column for that row arises from SQL's general rule that whenever an operand of a comparison is NULL, the result is unknown-and NULL, when it is the result of evaluating a Boolean expression, is considered synonymous with unknown. In fact, Figure gives a demonstration of the fact that SQL is not always faithful to its own concept, that NULL represents "a value exists here but we don't know which value". What U really means when it appears in the column for p <= q is that <= is undefined for that particular pair of truth values.


Related Discussions:- Conditionals - sql

Program, heap sort program in pl/sql

heap sort program in pl/sql

Data types in sql - timestamp, Data Types in SQL - Timestamp TIMESTAMP...

Data Types in SQL - Timestamp TIMESTAMP for values representing points in time on a specified uniform scale. DATE is used for timestamps on a scale of one day, such as DATE '2

What is a record, What Is a Record  ? A record is a group of related...

What Is a Record  ? A record is a group of related data items that stored in the fields, each with its own name and datatype. Assume that you have different data about an em

Using rename in combination with join - sql, Using RENAME in combination wi...

Using RENAME in combination with JOIN - SQL Example gives pairs of ids of students having the same name, by joining two renamings of IS_CALLED. Example gives an equivalent ex

Oracle development , Write a stored procedure that accepts the post code in...

Write a stored procedure that accepts the post code in which the customer resides as the input parameter. The procedure should then use an explicit cursor to display comprehensive

Keyword & parameter description - exceptions, Keyword & Parameter Descripti...

Keyword & Parameter Description: WHEN: This keyword introduces the exception handler. You can have many exceptions execute the similar sequence of the statements by follo

Difference between 9i & 10g, Difference between 9i & 10G When Oracle r...

Difference between 9i & 10G When Oracle releases any new databases then it are having some discrepancy with them. But 10G is having much difference than oracle 9i has. Oracle

Authorize and fetch data from instagram, Authorize and fetch data from Inst...

Authorize and fetch data from Instagram Project Description: Incorporate Instagram feed on mobile site platform: c#, ms sql, jquerymobile, jquery Web admin Author

Variable declaration - sql, Variable Declaration - SQL SQL's support f...

Variable Declaration - SQL SQL's support for variables is very similar to Tutorial D's, except that the syntax for creating persistent  variables-base tables-is quite differen

Cursor variables, Cursor Variables Similar to a cursor, cursor variable...

Cursor Variables Similar to a cursor, cursor variable points to the current row in the result set of a multi-row query. But, dissimilar a cursor, a cursor variable can be opene

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