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

Organic evolution, Organi c Evolution Evolution is a slow continuou...

Organi c Evolution Evolution is a slow continuous, irreversible and natural process of change to give rise to advance and diverse forms of life i.e. formation of new specie

Object types and collections - performance of application, Use Object Types...

Use Object Types and Collections The Collection types and object types increase your efficiency by allowing for the realistic data modeling. The Complex real-world entities an

Mysql developer/programmer, Project Description: I want somebody who can...

Project Description: I want somebody who can help me with an idea that I have been working on for a few months now. The person will require extensive knowledge of warcraft 3 PvP

Exceptions - syntax, Exceptions An exception is the runtime error or wa...

Exceptions An exception is the runtime error or warning condition that can be predefined or user-defined. The Predefined exceptions are raised implicitly through runtime system

I need sql to infopath data connection, I need SQL to infopath data connect...

I need SQL to infopath data connection Project Description: Want data retrieval connection from SQL to SharePoint infopath Skills required are Sharepoint, SQL

Packages, Packages The package is a schema object which groups logicall...

Packages The package is a schema object which groups logically associated to the PL/SQL items, types, and subprograms. The Packages have 2 sections: the specification & the bod

Creating a table, Creating a Table Syantax: CREATE TABLE ENROL...

Creating a Table Syantax: CREATE TABLE ENROLMENT (StudentId SID, Name   VARCHAR (30) NOT NULL, CourseId CID, PRIMARY KEY (StudentId, CourseId)) ; Explan

Definition of from - sql, Definition of FROM - SQL Recall that the ope...

Definition of FROM - SQL Recall that the operand of FROM is denoted by a commalist, each element of that commalist being a table expression optionally accompanied by a range v

Other monadic - sql, Other monadic - SQL In 2VL there are just 4 (2 2 ...

Other monadic - SQL In 2VL there are just 4 (2 2 ) monadic operators, of which negation is really the only "useful" one. When a third truth value is introduced we have 27 (3 3

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