Effects of null operator, PL-SQL Programming

Effects of NULL Operator

As a general rule-but not a universal one-if NULL is an argument to an invocation of a system-defined read-only operator, then NULL is the result of that invocation. As you can see, the code for HIGHER_OF includes A > B, an invocation of the system-defined read-only operator ">", which does follow the general rule. Hence, if NULL is substituted for either or both of the parameters A and B, then NULL- which in this case we can also call UNKNOWN because ">" is a Boolean operator-is the result of the invocation. You are perhaps now wondering how SQL handles the IF statement when the specified condition yields UNKNOWN: is the THEN clause evaluated, or is it the ELSE clause?

As you know, other programming languages are normally based on classical logic. In keeping with the existence of just two truth values, TRUE and FALSE, the syntax for IF statements (and IF expressions) in such languages has just the two forks, THEN for when the condition is TRUE, ELSE for when it is not (i.e., is FALSE). You might therefore reasonably expect a language that embraces n truth values to support a variety of IF that has n forks-under a language design principle that Fred Brooks referred to as conceptual integrity, which means adhering rigorously to the language's adopted concepts. Instead, SQL retains just the two forks, keeping the normal treatment of THEN as being the one for when the condition is TRUE and arbitrarily lumping UNKNOWN in with FALSE for the ELSE fork. You should now be able to see that the general rule ("NULL in, NULL out") for system-defined operators cannot be said to apply to user-defined ones. If A > B evaluates to UNKNOWN, then the result of the HIGHER_OF invocation is the argument substituted for B, which might or might not be NULL.

Posted Date: 1/18/2013 1:52:14 AM | Location : United States







Related Discussions:- Effects of null operator, Assignment Help, Ask Question on Effects of null operator, Get Answer, Expert's Help, Effects of null operator Discussions

Write discussion on Effects of null operator
Your posts are moderated
Related Questions
I need SQL Data Base Project Description: Network SQL database and SQL Setup two 4 workstation Skills required are Data Entry, MySQL, SQL

Parameter Aliasing   To optimize the subprogram call, the PL/SQL compiler can decide between the two techniques of the parameter passing. With the by-value techniques, the v

Updating a Variable Assignment of an attribute value in a variable of a structured type Synatx: SET SN.C = 'S2'; As in Example the entire statement is equivalent to a

Advantages of Wrapping   The PL/SQL Wrapper convert the PL/SQL source code into a transitional form of the object code. By hiding the application internals, the Wrapper secure

Explicit Cursors The set of rows returned by the query can include zero, one, or multiple rows, depending on how many rows meet your search criteria. Whenever a query returns

Create the four tables and populate them with the given data. Answer the following queries in SQL. 1. Get all part-color/part-city combinations. Note: Here and subsequently, the

Implicit Cursor Attributes The Implicit cursor attributes returns the information about the execution of an INSERT, DELETE, UPDATE, or SELECT INTO statement. The cursor attribu

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

Using %TYPE The %TYPE attribute gives the datatype of a variable or the database column. In the example below, the %TYPE gives the datatype of a variable: credit REAL(7,2); debi

Parameter and Keyword Description: collection_name: This keyword identifies the index-by table, nested table, or varray formerly declared within the present scope. cu