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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
CREATE FUNCTION ER
(SM ROW ( StudentId SID, Mark INTEGER ) MULTISET )
RETURNS TABLE (StudentId SID, Mark INTEGER)
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.
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Enrolment was split - SQL Example shows how relvars IS_CALLED and IS_ENROLLED_ON can be derived from the original ENROLMENT relvar, using projection in the initial assignment
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Assignment of Variable - Updating a Variable Syntax: SET SN = SID ('S2'); This can obviously be read as "set the variable SN to be equal in value to SID ( 'S2' )".
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