When are constraints checked - sql, PL-SQL Programming

Assignment Help:

When Are Constraints Checked

Under the model constraints are conceptually checked at all statement boundaries (and only at statement boundaries). By default the same is true of SQL. However, SQL does not support the "multiple assignment" concept, described in the theory book, for database updates. For that reason it has to include an alternative method of addressing the problems that multiple assignment addresses. SQL does so by allowing the checking of specified constraints to be temporarily deferred and reinstated later-but never across a transaction boundary. As a result, it is possible for the database to appear to be inconsistent, but only to the user whose as yet uncommitted transaction has given rise to that state of affairs. As a consequence of deferred constraint checking, SQL code that depends on consistency with declared constraints is obviously exposed to that assumption of consistency being false when the code is executed while checking is deferred.

For example, the table expression SELECT Name FROM IS_CALLED WHERE StudentId = 'S1' might be expected never to result in a table containing more than one row, thanks to the key constraint applying to IS_CALLED; thus it might be used in a scalar subquery. However, if the checking of that key constraint is temporarily deferred and two or more rows with StudentId equal to 'S1' temporarily appear in that table, then the scalar subquery will give rise to a run-time exception. Fortunately, SQL does allow a constraint to be declared as NOT DEFERRABLE, and that is the default option.


Related Discussions:- When are constraints checked - sql

Cursors in pl/sql, Cursors The Oracle uses work areas to execute the SQ...

Cursors The Oracle uses work areas to execute the SQL statements and to store process information. A PL/SQL construct known as the cursor. Let's you assume name a work area and

Using limit - collection method, Using LIMIT For nested tables, that h...

Using LIMIT For nested tables, that have no maximum size, the LIMIT returns NULL. For varrays, the LIMIT returns the maximum number of elements that a varray can have (that yo

Fetch statement - syntax, FETCH Statement The FETCH statement retrieve ...

FETCH Statement The FETCH statement retrieve rows of data one at a time from the result set of the multi-row query. The data is stored in fields or variables which correspond t

Rownum - sql pseudocolumns, ROWNUM The ROWNUM returns a number represe...

ROWNUM The ROWNUM returns a number representing the order in which a row was selected from the table. The first row selected has a ROWNUM of 1; the second row has a ROWNUM of

Oracle development , Write a stored procedure that accepts the post code in...

Write a stored procedure that accepts the post code in which the customer resides as the input parameter. The procedure should then use an explicit cursor to display comprehensive

Map and order methods, Map and Order Methods: The values of the scalar...

Map and Order Methods: The values of the scalar datatype like CHAR or REAL have a predefined order that allows them to be compared. While, the instances of an object type has

Using savepoint, Using SAVEPOINT The SAVEPOINT names and marks the pre...

Using SAVEPOINT The SAVEPOINT names and marks the present point in the processing of a transaction. Used with the ROLLBACK TO statement, the savepoints undo parts of a transac

Using extend - collection method, Using EXTEND To enlarge the size of ...

Using EXTEND To enlarge the size of a collection, use EXTEND. This process has 3 forms. The EXTEND appends one null element to a collection. And the EXTEND(n) appends n null e

Role of abstraction in pl/sql, Role of Abstraction in pl/sql: The abst...

Role of Abstraction in pl/sql: The abstraction is a high-level description or model of a real-world entity. The Abstractions keep our daily lives convenient. They help us ca

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

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