Image relations

TTM and its companion book DTATRM talk about an IMAGE RELATION function, that joins two relations, embedding matching tuples from the right as a relation-valued attribute of the left.

Here is a piece of code that creates an RVA. The relation exam_mark is partitioned across the several courses.

cer := course [{ CourseId, ExamResult := {{ * }} joinr exam_mark}]

CourseId   | ExamResult
----------------------------------
C1         | StudentId  | Mark
           | ---------------------
           | S1         |       85
           | S2         |       49
           | S4         |       93
C2         | StudentId  | Mark
           | ---------------------
           | S1         |       49
C3         | StudentId  | Mark
           | ---------------------
           | S3         |       66
C4         | StudentId  | Mark
           | ---------------------

Here is a piece of code that does the job of putting the tuples back together again.

cer [{ fold(union,ExamResult) }]

StudentId  | Mark
---------------------
S1         |       85
S2         |       49
S4         |       93
S1         |       49
S3         |       66

Note that this relies on an anonymous attribute, which is ‘lifted’ out as a single value.

Leave a Comment

Filed under Language

Leave a Reply

Your email address will not be published. Required fields are marked *