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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).
Package Body: The package specification is implemented by the package body. That is, the package body has the definition of every cursor and the subprogram declared in the pac
On occasion, some of Brewbean's customers mistakenly leave an item out of a basket already checked out, so they create a new basket containing the missing items. However, they requ
Managing Cursors The PL/SQL uses 2 types of cursors: implicit and explicit. The PL/SQL declares a cursor implicitly for all the SQL data manipulation statements, including th
Built-In Functions The PL/SQL provides a lot of powerful functions to help you to manipulate the data. These built-in functions fall into the categories as shown below: error r
1- You can check attribute names from each table in DBF11 by running for example: desc dbf11.Member; desc dbf11.Agent; desc dbf11.Producer; Because some attribute names in
Collections: The collection is an ordered group of elements, all of similar type (for illustration, the grades for a class of students). Each element has an exclusive subsc
Deriving Predicates from Predicates in SQL The corresponding section in the theory book describes how predicates can be derived from predicates using (a) the logical connectiv
Name Resolution During the compilation, the PL/SQL compiler relates identifiers like the name of a variable with an address or memory location, actual value, or datatype. Th
Name Resolution In potentially uncertain SQL statements, the names of the database columns take precedence over the names of the local variables and formal parameters. For e.g.
Implicit Cursor is declared and used by the oracle environment internally. while the explicit cursor is declared and used by the external user. more over implicitly cursors are no
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