Already have an account? Get multiple benefits of using own account!
Login in your account..!
Remember me
Don't have an account? Create your account in less than a minutes,
Forgot password? how can I recover my password now!
Enter right registered email to receive password!
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.
Projection and Existential Quantification - SQL Intuitively it might seem that projection in SQL is simply a matter of specifying the required columns in the SELECT clause, a
Example of NOT EXISTS in SQL Example: Use of NOT EXISTS CREATE ASSERTION Must_be_enrolled_to_take_exam_alternative1 CHECK ( NOT EXISTS (SELECT StudentId, CourseId
Explicit Cursor Attributes The cursor variable or each cursor has four attributes: %FOUND, %ISOPEN, %ROWCOUNT, and %NOTFOUND. When appended to the cursor or cursor variable, th
Parameter and Keyword Description: table_reference: This identifies a table or view which should be available when you execute the INSERT statement, and for that you sho
How Calls Are Resolved? The figure shows that how the PL/SQL compiler resolves the subprogram calls. When the compiler encounters the procedure or function call, it tries to di
Using TRIM This process has two forms. The TRIM removes an element from the end of the collection. The TRIM(n) removes the n elements from the end of the collection. For e.g.
Running the PL/SQL Wrapper To run the PL/SQL Wrapper, go through the wrap command at your operating system prompt by using the syntax as shown: wrap iname=input_file [oname=
Table Comparison - SQL The following definitions for relation comparisons: Let r1 and r2 be relations having the same heading. Then: r1 ⊆ r2 is true if every tuple of r1
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
Overloading The PL/SQL overloads the subprogram names. That is, you can use similar name for few different subprograms as long as their formal parameters differ in the number
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!
whatsapp: +91-977-207-8620
Phone: +91-977-207-8620
Email: [email protected]
All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd