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

Assignments in pl/sql, Assignments in pl/sql The Variables and constants...

Assignments in pl/sql The Variables and constants are initialized every time a block or subprogram is entered. By default, the variables are initialized to NULL. Therefore, unle

Using rollback - , Using ROLLBACK The ROLLBACK statements end the pres...

Using ROLLBACK The ROLLBACK statements end the present transaction and undo any change made during the transaction. The Rolling back is helpful for two reasons. Firstly, if yo

I want database development with analysis tools, Project Description: I ...

Project Description: I want a database for large governmental and private data sets on one country that will be easily extended to other countries in the future. Also, the datab

Other monadic - sql, Other monadic - SQL In 2VL there are just 4 (2 2 ...

Other monadic - SQL In 2VL there are just 4 (2 2 ) monadic operators, of which negation is really the only "useful" one. When a third truth value is introduced we have 27 (3 3

Need to change mysql query to pdo, Need to change mysql query to PDO Pro...

Need to change mysql query to PDO Project Description: I want someone to convert me 1 .php file that includes some sql/mysql stuff to PDO is a very small file. Skills requ

Assignment of variable - updating a variable, Assignment of Variable - Upda...

Assignment of Variable - Updating a Variable Syntax: SET SN = SID ('S2'); This can obviously be read as "set the variable SN to be equal in value to SID ( 'S2' )".

Processing transactions, Processing Transactions This part describes ho...

Processing Transactions This part describes how to do the transaction processing. You learn the fundamental techniques that safeguard the consistency of your database, involvin

Semidifference and not - sql, Semidifference and NOT - SQL In this sec...

Semidifference and NOT - SQL In this section first describe the relational difference operator, named MINUS. Example here shows SQL's closest counterpart of that operator.

Sql, Write a cursor to open an employee database and fetch the employee rec...

Write a cursor to open an employee database and fetch the employee record whose age is greater than 45

Referencing records, Referencing Records Unlike the elements in a coll...

Referencing Records Unlike the elements in a collection, that are accessed using subscripts, the fields in a record are accessed by name. To reference an individual field, you

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