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

Quantification in sql, Quantification in SQL To quantify something, as...

Quantification in SQL To quantify something, as the theory book has it, is to state its quantity, to say how many of it there are. For example, in Tutorial D the expression CO

Exception_init pragma - pl/sql, EXCEPTION_INIT Pragma The pragma EXCEPT...

EXCEPTION_INIT Pragma The pragma EXCEPTION_INIT relates an exception name with an Oracle error number. Which allow you to refer to any internal exception by the name and to wri

Advantages of wrapping, Advantages of Wrapping   The PL/SQL Wrapper co...

Advantages of Wrapping   The PL/SQL Wrapper convert the PL/SQL source code into a transitional form of the object code. By hiding the application internals, the Wrapper secure

Assigning and comparing collections, Assigning and Comparing Collections ...

Assigning and Comparing Collections One collection can be assigned to other by an SELECT, INSERT, UPDATE, or FETCH statement, an assignment statement, or by a subprogram call. A

Write sql queries, Write SQL queries to solve the following specifications....

Write SQL queries to solve the following specifications. Include the query AND THE OUTPUT.  A screen dump of the output is acceptable. Show as many rows as you can. A screen dump i

Row operators - sql operators, Row Operators The Row operators return ...

Row Operators The Row operators return or reference the particular rows. ALL retains the duplicate rows in the result of a query or in an aggregate expression. The DISTINCT el

%isopen - explicit cursor attributes, %ISOPEN The %ISOPEN yields TRUE ...

%ISOPEN The %ISOPEN yields TRUE if its cursor or cursor variable is open; or else, the %ISOPEN yields FALSE. In the illustration, you use the %ISOPEN to select an action:

Set operators - sql operators, Set Operators The Set operators combine...

Set Operators The Set operators combine the results of the two queries into one result. The INTERSECT returns all the distinct rows selected by both queries. The MINUS returns

Defining and declaring collections, Defining and Declaring Collections T...

Defining and Declaring Collections To create the collections, you must define a collection type, and then declare the collections of that type. You can define the VARRAY types a

Example of except operator - sql, Example of EXCEPT Operator - SQL Ex...

Example of EXCEPT Operator - SQL Example, like its counterpart in the theory book, illustrates the convenience of allowing any table expression to be the source for an INSERT

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