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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, as in Example.
Example: Projection (incorrect)
Student StudentId is enrolled on some course.
SELECT StudentId FROM IS_ENROLLED_ON
Unfortunately, though, if some student is currently enrolled on more than one course (as indeed student S1, Anne, is in our example database), then the row giving that student's id appears twice in the result, which because of that duplicate appearance does not represent a relation. To avoid multiple appearances of the same row SQL requires you to write the word DISTINCT. (The key word ALL can be given instead of DISTINCT, clarifying that duplicate rows are not to be eliminated. As ALL is the default option).
Closing a Cursor The CLOSE statements disable the cursor, and the result set becomes undefined. An illustration of the CLOSE statement as shown: CLOSE c1;
Cause of Indeterminacy in SQL One root cause of indeterminacy in SQL lies in its implementation of comparison for equality. For certain system-defined types it is possible for
Declaring a Cursor The Forward references are not allowed in the PL/SQL. Therefore, you must declare a cursor before referencing it in other statements. Whenever you declare a
Biconditional - SQL The biconditional p ↔ q can be expressed in Tutorial D by p = q and the same is true of SQL. The question then arises as to whether, in SQL, p = q is equiv
Transaction Control The Oracle is transaction oriented; that is, Oracle uses the transactions to make sure the data integrity. The transaction is a sequence of SQL data manip
Parameter Default Values As the illustration below shows, you can initialize the IN parameters to the default values. In that way, you can pass various numbers of actual par
Keyword and Parameter Description: label_name: This is an undeclared identifier which optionally labels the PL/SQL block. When used, label_name should be enclosed by the do
UNION and OR - SQL SQL supports UNION explicitly but differently from the way it supports JOIN explicitly. As we have seen, JOIN is used exclusively within the FROM clause, su
Using DEFAULT You can use the keyword DEFAULT rather than that of the assignment operator to initialize the variables. For e.g. the declaration blood_type CHAR := ’O’; it can b
Updating by insertion Syntax : INSERT INTO ENROLMENT VALUES (SID ('S4'), 'Devinder', CID ('C1'));
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