Example of groupby operator - sql, PL-SQL Programming

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Example of GROUPBY Operator

Example: How many students sat each exam,

using GROUP BY, NATURAL LEFT JOIN, and COALESCE

SELECT CourseId, COALESCE (n, 0) AS n

FROM COURSE NATURAL LEFT JOIN

(SELECT CourseId, COUNT (*) AS n

FROM EXAM_MARK

GROUP BY CourseId) AS T

Explanation

NATURAL JOIN: Note, however, that the use of LEFT makes this an outer join, whereas Codd's term natural join referred to the "inner" variety only.
LEFT specifies that each unmatched row in the first join operand, COURSE, is to be extended with NULL for the column n.
COALESCE (n, 0) AS n effectively replaces those appearances of NULL by the correct value, 0.

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