Example of not exists operator - sql, PL-SQL Programming

Example of NOT EXISTS Operator - SQL

Example is a translation into SQL of the corresponding example, which is included there merely to show that for any scalar comparison there is an alternative formulation using IS_EMPTY.

Example: MAX_ENROLMENTS expressed using an invocation of NOT EXISTS

CREATE ASSERTION MAX_ENROLMENTS_alternative1

CHECK (NOT EXISTS (SELECT *

FROM (VALUES (SELECT COUNT (*)

FROM IS_ENROLLED_ON)) AS

V (N)

WHERE V.N > 20000));

Explanation

  • VALUES ( SELECT COUNT(*) FROM IS_ENROLLED_ON ) denotes a table with just one column, unnamed, in whose single row the value of that column is the number of rows in the current value of IS_ENROLLED_ON. The SELECT expression is parenthesized to make it into a scalar subquery and given as the argument to an invocation of VALUES, which makes the number denoted by that scalar subquery into a one-row, one-column table. (Actually, it might be safer to place an extra pair of parentheses around the SELECT expression here. Although VALUES 1 and VALUES (1) are equivalent, it might not be clear as to which role the single parentheses are taking: do they denote a scalar subquery, as I have assumed, or are they the optional ones surrounding a single table expression? If the latter, we would expect a syntax error.)
  • AS V (N) defines the range variable V to refer to what in this case is just the single row of that table, and also assigns the name N to its only column.
  •  WHERE V.N > 20000 operates on that one-row, one-column table to yield a table of heading (N INTEGER) that is empty if and only if the single row in that one-row, one-column table fails to satisfy the condition N > 20000. Thus, the result is empty only when the number of enrolments is in fact no greater than the maximum allowed.
Posted Date: 1/18/2013 7:44:45 AM | Location : United States







Related Discussions:- Example of not exists operator - sql, Assignment Help, Ask Question on Example of not exists operator - sql, Get Answer, Expert's Help, Example of not exists operator - sql Discussions

Write discussion on Example of not exists operator - sql
Your posts are moderated
Related Questions
SQL Database: So, an SQL database is one whose symbols are organized into a collection of tables. Now, shows an SQL table as the current value of an SQL variable, ENROLMENT, b

IF Statement The IF statement executes a series of statement conditionally. Whether the series is executed or not depends on the value of the Boolean expression. Syntax:

Transaction Visibility As the figure shows, the changes made by an autonomous transaction become visible to another transaction whenever the autonomous transaction commits. Th

Packages The package is a schema object which groups logically associated to the PL/SQL items, types, and subprograms. The Packages have 2 sections: the specification & the bod

Forward Declarations The PL/SQL needs that you declare an identifier before using it. And hence, you should declare a subprogram before calling it. For illustration, the decla

Perform the following queries on the Hospital1.DB using SQL Anywhere (START EARLY!). a. Which patients have purchased the drug "Tylenol"? List the names and addresses. Arrange the

Bulk Binds advantages In the Embedded Oracle RDBMS, the PL/SQL engines accept any valid PL/SQL subprogram or block. As the figure shows, the PL/SQL engine executes all procedur

Using LIMIT For nested tables, that have no maximum size, the LIMIT returns NULL. For varrays, the LIMIT returns the maximum number of elements that a varray can have (that yo

Relational Operators and Logical Operators It prepares the ground for subsequent sections in which each specific relational operator is paired with its logical counterpart, su

CURRVAL and NEXTVAL The series is a schema object which generates the sequential numbers. Whenever you form a sequence, you can specify its primary value and an increment. T