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

Library system, Hi,am developing a library system and relating all the tabl...

Hi,am developing a library system and relating all the table is somehow complex,could you kindly assist me

Fetching across commits, Fetching Across Commits The FOR UPDATE clause...

Fetching Across Commits The FOR UPDATE clauses acquire exclusive all row locks. All rows are locked when you open the cursor, and when you commit your transaction they are unl

Write an anonymous block that contains a pl/sql function, a. Write an anon...

a. Write an anonymous block that contains a PL/SQL function. Given an order number orderNo, the function will calculate the total number of the parts in the order. Then the anonym

While-loop - iterative control, WHILE-LOOP The WHILE-LOOP statement rela...

WHILE-LOOP The WHILE-LOOP statement relates a condition with the series of statements enclosed by the keywords LOOP and END LOOP, as shown: WHILE condition LOOP sequence_of_sta

Cursor variables in pl sql, Cursor Variables:   To execute the multi-...

Cursor Variables:   To execute the multi-row query, the Oracle opens an unnamed work region that stores the processing information. You can use an explicit cursor that names

Delete command - sql, DELETE Command - SQL Loosely speaking, DELETE re...

DELETE Command - SQL Loosely speaking, DELETE removes some existing rows from its target table. Suppose the university decides that course C3 is to be withdrawn. Example shows

Second step at defining type sid in sql, Second Step at defining type SID i...

Second Step at defining type SID in SQL CREATE TYPE SID AS VARCHAR(5) ; Explanation: TYPE SID announces that a type named SID is being defined to the system.

Create a procedure to tax calculation task, Complete the following steps to...

Complete the following steps to create a procedure to calculate the tax on an order. The BB_TAX table contains the states that require taxes to be submitted for Internet sales. If

%notfound - explicit cursor attributes, %NOTFOUND The %NOTFOUND is log...

%NOTFOUND The %NOTFOUND is logical, opposite of the %FOUND. The %NOTFOUND yields FALSE if the last fetch returned a row, or TRUE when the final fetch failed to return a row. I

Implicit rollbacks, Implicit Rollbacks Before execute the INSERT, UPDA...

Implicit Rollbacks Before execute the INSERT, UPDATE, or DELETE statement, the Oracle marks an implicit savepoint . When the statement fails, the Oracle rolls back to the save

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd