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 to those relvars. Here is how the same effect can be achieved in SQL:
Example: Splitting ENROLMENT
CREATE TABLE IS_CALLED
AS (SELECT DISTINCT StudentId, Name FROM ENROLMENT)
WITH DATA;
ALTER TABLE IS_CALLED ADD CONSTRAINT PRIMARY KEY ( StudentId );
CREATE TABLE IS_ENROLLED_ON
AS (SELECT DISTINCT StudentId, CourseId FROM ENROLMENT)
WITH DATA;
ALTER TABLE ADD CONSTRAINT PRIMARY KEY ( StudentId, CourseId ) ;
DROP TABLE ENROLMENT;
Explanation:
- CREATE TABLE IS_CALLED announces that what follows defines a base table named IS_CALLED.
- AS (SELECT DISTINCT StudentId, Name FROM ENROLMENT) specifies that the columns of ENROLMENT and their declared types are as in the specified expression.
- WITH DATA additionally specifies that the table resulting from the specified expression is to be the initial value of IS_CALLED.
- ALTER TABLE IS_CALLED ADD PRIMARY KEY ( StudentId ) specifies a constraint to the effect that no two distinct rows having the same StudentId value can ever appear simultaneously in IS_CALLED. Note that this constraint has to be given as a separate statement from the one that creates the base table. If the key word DISTINCT had been omitted, the CREATE TABLE statement would have succeeded but the ALTER TABLE statement would have failed because the required constraint would have been violated by the two appearances of the row for student S1, Anne.
- Similar comments apply to the CREATE and ALTER TABLE statements for IS_ENROLLED_ ON, but in the equivalent example noted that the specification KEY {StudentId, CourseId}, required by Tutorial D, is theoretically redundant because the entire heading is always a superkey. Here, the corresponding ALTER TABLE statement is not redundant because in the absence of any key constraints SQL allows the same row to appear several times simultaneously in the same base table.
- DROP TABLE ENROLMENT destroys the variable we have no further use for.