Use of table expressions - expressing constraint conditions, PL-SQL Programming

Assignment Help:

Use of Table Expressions - Expressing Constraint Conditions

With the exception of key constraints, the examples in the theory book all explicitly reference at least one relvar and thus involve invocations of relational operators or aggregate operators. Assuming support for CREATE ASSERTION, we can always derive SQL counterparts of these examples using table expressions and truth-valued operators, but when that assumption does not hold we need to look for alternative solutions using table constraints. In most cases these will entail the use of subqueries and even that technique is prohibited by many implementations. In some cases special syntactic constructs are available, as we shall see, but there are several for which no SQL solution is available unless the implementation supports CREATE ASSERTION or subqueries in table constraints.Now, the reason usually given for lack of support for subqueries in constraints is that in general such expressions can require the DBMS to examine the entire content of possibly very large tables.

If database updates are expected to occur frequently-and are perhaps required to occur very frequently indeed- then declaration of such constraints would give rise to an intolerable slowing down of the updating process. Of course this is an extremely valid concern and we have to admit that integrity might occasionally have to be compromised for performance reasons, but consider the user with a small database that is subject to comparatively infrequent updating but nevertheless has strong integrity requirements. Might not such a user feel unfairly treated by a system that prohibits the declaration of required constraints? Defenders of the status quo respond to this argument by holding that language constructs that can give rise to disappointment for performance reasons, to such an extent as to militate against their use in common practical situations, should be banned. But sometimes users resort to implementing constraints, as best they can, in application code when they wish to enforce a constraint that is not supported by the DBMS but nevertheless does not adversely impair performance. The DBMS could almost certainly enforce such constraints much more efficiently and much more reliably. We can also point to various other SQL constructs that might be subject to similar concerns but are supported nonetheless. For example, if tables T1, T2, and T3 each contain 100,000 rows, then SELECT * FROM T1, T2, T3, when evaluated, delivers a table containing a quadrillion rows.


Related Discussions:- Use of table expressions - expressing constraint conditions

Special cases of projection, Special cases of projection This section ...

Special cases of projection This section describes the identity projection, r {ALL BUT}, and the projection on no attributes, r { }, which yields TABLE_DUM when r is empty, ot

Scope and visibility- pl/sql, Scope and Visibility The References to an ...

Scope and Visibility The References to an identifier are resolved according to its visibility and scope. The scope of an identifier is that area of a program unit (subprogram, b

Character types in pl/sql, Character Types The Character types allow yo...

Character Types The Character types allow you to store alphanumeric data, represent words and text, and manipulate the character strings. CHAR You use the CHAR dataty

%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

Variable declaration - sql, Variable Declaration - SQL SQL's support f...

Variable Declaration - SQL SQL's support for variables is very similar to Tutorial D's, except that the syntax for creating persistent  variables-base tables-is quite differen

Keyword & parameter description - exception_init pragma, Keyword & Paramete...

Keyword & Parameter Description: PRAGMA: These keywords signify that the statement is a pragma (i.e. compiler directive). The Pragmas are processed at the compile time, n

Example of foreign key constraint - sql, Example of Foreign Key Constraint ...

Example of Foreign Key Constraint Example: Alternative formulation for 6.3 as a foreign key constraint ALTER TABLE EXAM_MARK ADD CONSTRAINT Must_be_enrolled_to_take_exam

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

Package standard, Package STANDARD The package named STANDARD defines t...

Package STANDARD The package named STANDARD defines the PL/SQL atmosphere. The package specification globally declares the exceptions, types, and subprograms that are available

Using a host variable, Using a Host Variable You can declare the curso...

Using a Host Variable You can declare the cursor variable in the PL/SQL host environment like an OCI or Pro C program. To use the cursor variable, you should pass it as a host

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