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

Ask an Expert

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

Write your message here..

Related Questions

Using aggregation on nested tables - sql, Using Aggregation on Nested Table... Using Aggregation on Nested Tables Example is the most direct translation of its counterpart in the theory book that can be obtained in SQL but it is so over-elaborate that no

Use the nocopy compiler hint - performance of application, Use the NOCOPY C... Use the NOCOPY Compiler Hint By default, the OUT and IN OUT parameters are passed by the value i.e. the value of an IN OUT actual parameter is copied into the corresponding fo

Differentiate between snowflake schema and star schema, Problem: (a) De... Problem: (a) Define the following terms: (i) data mining. (ii) OLAP. (b) Differentiate between snowflake schema and star schema. Support your answer with appropriate

Inner join, Inner Join We have learned how to retrieve data from one t... Inner Join We have learned how to retrieve data from one table by using SELECT statement. But, as we have learned, normalized relational databases mean the data is spread betw

Name resolution-naming conventions, Name Resolution In potentially uncer... Name Resolution In potentially uncertain SQL statements, the names of the database columns take precedence over the names of the local variables and formal parameters. For e.g.

Example of check constraints - sql, Example of Check Constraints Examp... Example of Check Constraints Example: Workaround for when subqueries not permitted in CHECK constraints CREATE FUNCTION NO_MORE_THAN_20000_ENROLMENTS ( ) RETURNS BOOLEAN

Cursor variables, What Are Cursor Variables  ? The Cursor variables ar... What Are Cursor Variables  ? The Cursor variables are like C or Pascal pointers that hold the memory location (address) of some item rather of the item itself. Therefore, decl

Primary key, PRIMARY KEY: PRIMARY KEY  indicates that the table is sub... PRIMARY KEY: PRIMARY KEY  indicates that the table is subject to a key constraint, in this case declaring that no two rows in the table assigned to ENROLMENT can ever have the

Control structure, Control Structures The Control structures are the mo... Control Structures The Control structures are the most important PL/SQL extension to the SQL. Not only does PL/SQL let you manipulate Oracle data, it lets you process the data

Design a script and integrate procedures, Initial thought process: Design... Initial thought process: Design a script which was simple and user friendly. Integrate procedures/functions to extract data under the hood. I focused on giving the user the opt