Example of add constraint in sql, PL-SQL Programming

Assignment Help:

Example of ADD CONSTRAINT in SQL

Example: Alternative formulation for MAX_ENROLMENTS

ALTER TABLE IS_ENROLLED_ON

ADD CONSTRAINT MAX_ENROLMENTS

CHECK ((SELECT COUNT (*) FROM IS_ENROLLED_ON) <= 20000);

Explanation

ALTER TABLE IS_ENROLLED_ON announces that an alteration to the definition of base table IS_ENROLLED_ON is being specified.
ADD CONSTRAINT MAX_ENROLMENTS states that the alteration in question is the addition of something SQL calls a table constraint, and its name is MAX_ENROLMENTS. A table constraint is a condition that is required to be satisfied by every row appearing in the base table for which that constraint is defined. Thus, in general, it is an open expression of the kind that can appear as the condition of a WHERE clause. In this example the condition is in fact a closed expression-it contains no reference to a column of IS_ENROLLED_ON-and thus if it is satisfied by one row of that table, then it is satisfied by all of them.
A table constraint is one that must be satisfied by every row of the applicable table, it is always satisfied when the applicable table is empty-there is no row for which the constraint fails. In the case at hand, this is not a problem, because obviously, when IS_ENROLLED_ON is empty, then its cardinality-zero-does not exceed 20,000. However, suppose a constraint was required to the effect that IS_ENROLLED_ON must never be empty. That could be achieved by changing <= 20000 to > 0 , but the same change to Example would be ineffectual: when IS_ENROLLED_ON is empty, it contains no row that fails to satisfy the constraint. That is why SQL is incomplete with respect to database integrity when support for CREATE ASSERTION is absent.

Related Discussions:- Example of add constraint in sql

Creating a sql file, Creating a SQL file 1. Open a new file in Notepad...

Creating a SQL file 1. Open a new file in Notepad++ and save it to the location c:\mysql\bin, with the name lab8script.sql (the file extension should be .sql ). Add a MySQL co

Create Tables, Hi there, I have the final part of a submission to do, it ...

Hi there, I have the final part of a submission to do, it is a demonstration that takes place tomorrow. I do not have to use previous information, but i have resources that sho

Projection in sql - correct version, Projection in SQL - correct version ...

Projection in SQL - correct version Student StudentId is enrolled on some course. SELECT DISTINCT StudentId FROM IS_ENROLLED_ON In more complicated examples it is someti

Short-circuit evaluation-pl/sql expressions , Short-Circuit Evaluation ...

Short-Circuit Evaluation When computing a logical expression, the PL/SQL uses short-circuit evaluation. That is, the PL/SQL stops computing the expression as soon as the result

Iterative control:exit statements, EXIT The EXIT statement forces a loop...

EXIT The EXIT statement forces a loop to done unconditionally. Whenever an EXIT statement is encountered, the loop is done immediately and controls the passes to the next statem

Using a host variable, Using a Host Variable You can declare the curso...

Using a Host Variable You can declare the cursor variable in the PL/SQL host environment like an OCI or Pro C program. To use the cursor variable, you should pass it as a host

Closing a cursor variable, Closing a Cursor Variable The CLOSE stateme...

Closing a Cursor Variable The CLOSE statement disables the cursor variable. After that, the related result set is undefined. The syntax for the same is as shown below: CLOS

Execute privilege, EXECUTE Privilege To call an invoker-rights routine ...

EXECUTE Privilege To call an invoker-rights routine straightforwardly, the users should have the EXECUTE privilege on that routine. By yielding the privilege, you permit a user

Deleting objects in pl sql, Deleting Objects You can use the DELETE st...

Deleting Objects You can use the DELETE statement to eradicate objects from an object table. To eradicate objects selectively, you use the WHERE clause, as shown below: BEG

Mutual recursion, Mutual Recursion The Subprograms are mutually recursi...

Mutual Recursion The Subprograms are mutually recursive if they directly or indirectly call each other. In the illustration below, the Boolean functions odd & even, that dete

Write Your Message!

Name

Email id

Message

Verfication Code

User Account

All Pages