Example of tables within a table - sql, PL-SQL Programming

Assignment Help:

Example of Tables within a Table - SQL

Example: Obtaining C_ER from COURSE and EXAM_MARK

SELECT CourseId, CAST (TABLE (SELECT DISTINCT StudentId, Mark FROM EXAM_MARK AS EM WHERE EM.CourseId = C.CourseId)

AS ROW (StudentId SID, Mark INTEGER) MULTISET)

AS ExamResult FROM COURSE AS C

Explanation

  • The SELECT clause operates on each row of the result of the FROM clause-i.e., on each row of the COURSE table, deriving two columns, CourseId and ExamResult.
  • CourseId is self-explanatory, merely carrying forward the column values from the column of that name in COURSE.
  • TABLE ( SELECT DISTINCT StudentId, Mark FROM EXAM_MARK AS EM WHERE EM.CourseId = C.CourseId ) denotes a multiset whose elements are rows, obtained by taking the StudentId and Mark values from those rows of EXAM_MARK that match the current row of COURSE on CourseId. Note very carefully, however, that this multiset does not necessarily inherit the column names, StudentId and Mark, from the table that is the operand to the invocation of TABLE. The SQL standard allows the column names to be "implementation-dependent" (i.e., undefined) so long as no two columns have the same name. An implementation that nevertheless carried forward the unique names StudentId and Mark would be both sensible and conforming, and would obviate the need for the CAST invocation explained in the next bullet. The same multiset would result if the word DISTINCT had been omitted, thanks to the WHERE condition, but I include it because the example in the theory book uses COMPOSE, which is defined as a projection of a join, and SQL's counterpart of projection uses SELECT DISTINCT.
  • CAST (t AS ROW ( StudentId SID, Mark INTEGER ) MULTISET ), where t is the above TABLE expression, addresses the aforementioned possible problem by assigning the required column names. Note that we need to know and write down the declared types of those columns as well as their names. The "type conversion" operator CAST. Here it is being used to convert a value of some incompletely defined multiset type to one whose multiset type is explicitly defined.
  • AS ExamResult then gives the resulting column the name ExamResult. Note that here the name comes after AS and the expression defining it comes before, in the same style as the use of AS to define the range variables C and EM in the example.

The values for columns such as ExamResult in this example have sometimes been referred to informally as nested tables, being "tables within a table", so to speak. Unfortunately, however, they are not actually tables, but rather multisets of rows. Because of that fact, a column such as ExamResult cannot appear as an element in a FROM clause.


Related Discussions:- Example of tables within a table - sql

Defining autonomous transactions, Defining Autonomous Transactions To ...

Defining Autonomous Transactions To define an autonomous transaction, you use the pragma (compiler directive) AUTONOMOUS_TRANSACTION. The pragma instructs the PL/SQL compiler

Components of an object type in pl/sql, Components of an Object Type: A...

Components of an Object Type: An object type encapsulates the operations and data. Therefore, you can declare the methods and attributes in an object type specification, but no

Exception_init pragma - pl/sql, EXCEPTION_INIT Pragma The pragma EXCEPT...

EXCEPTION_INIT Pragma The pragma EXCEPTION_INIT relates an exception name with an Oracle error number. Which allow you to refer to any internal exception by the name and to wri

Joining in sql, Joining in SQL Joining IS_CALLED and IS_ENROLLED_ON in...

Joining in SQL Joining IS_CALLED and IS_ENROLLED_ON in SQL SELECT * FROM IS_CALLED NATURAL JOIN IS_ENROLLED_ON This is an example of an SQL table expression. I have been

Deleting objects in pl sql, Deleting Objects You can use the DELETE st...

Deleting Objects You can use the DELETE statement to eradicate objects from an object table. To eradicate objects selectively, you use the WHERE clause, as shown below: BEG

Keyword and parameter description - goto statement, Keyword and Parameter D...

Keyword and Parameter Description: label_name: This is an undeclared identifier which labels an executable statement or the PL/SQL block. You can use a GOTO statement to

Triggers, At times, customers make mistakes in submitting their orders and ...

At times, customers make mistakes in submitting their orders and call to cancel the order. Brewbean’s wants to create a trigger that automatically updates the stock level of all pr

Running the pl/sql wrapper, Running the PL/SQL Wrapper To run the PL/SQ...

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=

Calling constructors in pl sql, Calling Constructors: The Calls to a c...

Calling Constructors: The Calls to a constructor are allowed wherever the function calls are allowed. Similarly to the functions, a constructor is called as a section of an ex

Indeterminacy in sql, Indeterminacy in SQL Some SQL expressions are ac...

Indeterminacy in SQL Some SQL expressions are actually not function invocations at all in the mathematical sense, being indeterminate-invocations operating on identical input

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