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Semijoin and Composition - SQL
For semijoin, the dyadic relational operator MATCHING, defined thus:
r1 MATCHING r2, where r1 and r2 are relations such that r1 JOIN r2 is defined, is equivalent to
(r1 JOIN r2) {r1-attrs}
where r1-attrs is a commalist containing all and only the attribute names of r1.
and the example
COURSE MATCHING EXAM_MARK
is given as a relational expression for the predicate, "There exist a student StudentId and a mark Mark such that StudentId sat the exam and scored Mark marks for course CourseId and CourseId is entitled Title" (which could be abbreviated to "At least one student sat the exam for Course CourseId, entitled Title"). The resulting relation consists of just those tuples in COURSE that have at least one matching tuple in EXAM_MARK.
Definition of CROSS JOIN - SQL Let s = t1 CROSS JOIN t2, where t1 and t2 are table expressions optionally accompanied by range variables. Then: Note: Here T denotes Table
%ISOPEN The %ISOPEN yields TRUE if its cursor or cursor variable is open; or else, the %ISOPEN yields FALSE. In the illustration, you use the %ISOPEN to select an action:
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Example of NOT EXISTS Operator - SQL Example is a translation into SQL of the corresponding example, which is included there merely to show that for any scalar comparison the
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 COURS
Comparison Operators Usually, you use the comparison operators in the WHERE clause of a data manipulation statement to form the predicates, that compare one expression to anot
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