Example of using aggregation on nested tables, PL-SQL Programming

Example of Using Aggregation on Nested Tables

Example: How many students sat each exam

WITH C_ER AS (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) SELECT CourseId, (SELECT COUNT (*)  FROM TABLE (ER (ExamResult)) AS t) AS n FROM C_ER

Explanation

  • The WITH clause, occupying the first nine lines of the example, illustrates SQL's counterpart of Tutorial D's construct of the same name. It assigns the name, C_ER, to the result of Example. That name, C_ER, is then used in the FROM clause of the expression that follows the WITH clause. Note that here the name comes before AS and the expression defining it comes after. This is consistent with the analogous use of AS in CREATE VIEW statements.
  • TABLE(ER(ExamResult)) seems to be the only way of having a multiset valued column operated on as an element of a FROM clause-a simple column name is not allowed to appear here. TABLE(ExamResult) can't be used either, because when an invocation of TABLE appears as a FROM clause element, its operand is required to be, specifically, an invocation of a user-defined function. Assumed here ER defined like:

CREATE FUNCTION ER

(SM ROW ( StudentId SID, Mark INTEGER ) MULTISET )

RETURNS TABLE (StudentId SID, Mark INTEGER)

RETURN SM;

The type name TABLE ( StudentId SID, Mark INTEGER ) is actually just a synonym for ROW (StudentId SID, Mark INTEGER ) MULTISET). The misleading synonym is available only in a RETURNS clause and not as a parameter type, for example. So ER is actually a no-op, returning its input.

  • (SELECT COUNT(*) FROM TABLE(ER(ExamResult)) AS t) is a scalar subquery, yielding the cardinality of the multiset of rows that is the value of the column ExamResult in the current row of C_ER. Because we are using the expression to denote a scalar value rather than a table, naming the column would be pointless (apart, perhaps, from injecting a somewhat sarcastic element of purism). As COUNT(*) doesn't use a column name, Example is valid even if we omit the invocation of CAST to assign column names.
  • AS n then gives the resulting column the name n. 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.
Posted Date: 1/18/2013 6:18:21 AM | Location : United States







Related Discussions:- Example of using aggregation on nested tables, Assignment Help, Ask Question on Example of using aggregation on nested tables, Get Answer, Expert's Help, Example of using aggregation on nested tables Discussions

Write discussion on Example of using aggregation on nested tables
Your posts are moderated
Related Questions
Using RENAME in combination with JOIN - SQL Example gives pairs of ids of students having the same name, by joining two renamings of IS_CALLED. Example gives an equivalent ex

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

Write a query to find academics that are authors and that have only ever coauthored papers with authors from institutes in the same state as their own. List their academic number,

DELETE Statement The DELETE statement eliminates whole rows of data from the specified table or view. Syntax:

Data Types in SQL - Interval, Boolean INTERVAL for values denoting, not intervals (!) but durations in time, such as 5 years, 3 days, 2 minutes, and so on. BOOLEAN, con

Third Step at defining type SID in SQL CREATE DOMAIN SID AS VARCHAR(5) CHECK ( VALUE IS NOT NULL AND SUBSTRING(VALUE FROM 1 FOR 1) = 'S' AND CAST('+'||SUBSTRING(VALUE

Using Pragma RESTRICT_REFERENCES: The function called from the SQL statements should obey certain rules meant to control the side effects. To check for violation of the rules,

Cursors   To execute the multi-row query, the Oracle opens an unnamed work region which stores the processing information. The cursor names the work region, access the informa

Functions The function is a subprogram which can take parameters and be invoked. Normally, you can use a function to calculate a value. The function has 2 sections: the specifi

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