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Example of Cast Operator
So long as CAST is used as shown, we could obtain the total marks for each exam in similar fashion, using SUM (Mark) AS TotalMarks. However, this gives NULL, instead of zero, for the courses whose exams nobody sat. That problem can be addressed by using COALESCE, as shown in Example.
Example: Give the total of marks for 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 SUM (Mark)
FROM TABLE(ER (ExamResult)) AS t) AS n
FROM C_ER
Explanation
Scoping Within the similar scope, all the declared identifiers should be unique. So, even if their datatypes differ, the variables and parameters cannot share the similar name.
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which operation is used if we are interested in only certain columns of a table?
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