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

Package body, The Package Body The package specification is implemented...

The Package Body The package specification is implemented by the package body. That is, the package body has the definition of every cursor and the subprogram declared in the p

Use native dynamic sql - improve performance of application, Use Native Dyn...

Use Native Dynamic SQL A few programs (a normal-purpose report writer for illustration) should build and process a variety of SQL statements at run time. Therefore, their full

Level - sql pseudocolumns, LEVEL You use the LEVEL with the SELECT CON...

LEVEL You use the LEVEL with the SELECT CONNECT BY statement to categorize rows from a database table into a tree structure. The LEVEL returns the level number of a node in a

Delimiters, Delimiters A delimiter is a simple or compound symbol whi...

Delimiters A delimiter is a simple or compound symbol which has a special meaning to PL/SQL. For example, you use delimiters to symbolize an arithmetic operation like additio

Using lock table, Using LOCK TABLE You use the LOCK TABLE statement to...

Using LOCK TABLE You use the LOCK TABLE statement to lock the whole database tables in the specified lock mode so that you can share or deny the access to them. For illustrati

Parameter and keyword description - select into statement, Parameter and Ke...

Parameter and Keyword Description: select_item: This select_item is a value returned by the SELECT statement, and then assigned to the equivalent variable or field in the

Synonyms- naming conventions, Synonyms You can create the synonyms to pr...

Synonyms You can create the synonyms to provide location transparency for the remote schema objects like tables, views, sequences, stand-alone subprograms, and packages. Though,

Using cursor attributes - bulk bind performance improvement, Using Cursor A...

Using Cursor Attributes To process the SQL data manipulation statements, the SQL engine must opens an implicit cursor named SQL. This cursor's attributes (%FOUND, %NOTFOUND, %

Assignment 4, I need a query for PL/SQL, selecting names with cursor, goes ...

I need a query for PL/SQL, selecting names with cursor, goes down the list, assigns usernames (initials001) based on initials in the name. If two names have same initials the user

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