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
i NEED THE QURIES TO SOME OF THE PROBLEMS USING plsql

Important Distinctions The list of important distinctions are given below: Value versus variable Syntax versus semantics Variable versus variable reference

Using ROLLBACK The ROLLBACK statements end the present transaction and undo any change made during the transaction. The Rolling back is helpful for two reasons. Firstly, if yo

Read-Only Operator (+) - SQL The term read-only operator to the mathematical term function. Here I just need to add that the SQL standard reserves the term function for read-

Demonstrate your knowledge of PL/SQL programming by writing and thoroughly testing triggers and stored procedures associated with an e-commerce application that provides security l

Logical Connectives - SQL SQL's extended truth tables in which the symbol, for unknown, appears along with the usual T and F. Negation (NOT, ¬) Conjunction (

Definition of CROSS JOIN - SQL Let s = t1 CROSS JOIN t2, where t1 and t2 are table expressions optionally accompanied by range variables. Then: Note: Here T denotes Table

heap sort program in pl/sql

Expressions   An expression is a randomly complex combination of the constants, variables, literals, operators, & function calls. The simplest expression is the single variabl

Defining Autonomous Transactions To define an autonomous transaction, you use the pragma (compiler directive) AUTONOMOUS_TRANSACTION. The pragma instructs the PL/SQL compiler