On 2016-03-07 4:25 AM, Paul Bennett wrote:
I have most of a spec (and a tiny bit of pseudocode) prepared for
adding operator overloads to Result Sets, to make it easier to write &
reuse pieces of queries in a style more like Relational Algebra &
Tuple-Relational Calculus. For instance, '/' for relational division,
'*' for fiber product, 'x' for explicit cross join, '|' for left
semijoin, '||' for left outer join, '.' for full outer join, '>' for
left antijoin, '+' for union, '&' for intersection, '-' for "except",
and so on.
Paul,
Firstly: What are the exact semantics you would use for your relational
division, since there is more than one operation historically that has been
given that name? I mean, express it in terms of other relational
algebra/calculus. On a related note, it has been argued that this operation
isn't very useful in practice; do you see a use for it or are you just being a
completionist?
Secondly: Have you considered using other symbols for these operators? The
ones you chose don't in my mind bear much resemblance to their meaning, or I
otherwise prefer not to use math operators for set operations.
I recommend you use short alphanum names instead, or Unicode symbols, like this:
⊎ addition
∖ except
∩ intersect
∪ union
∆ symmetric difference
⨝ natural join (or ⋈ alternately)
⋊ semijoin
⋉ semijoin
⊿ antijoin
⨯ cross product
⟕ half outer join
⟖ half outer join
⟗ full outer join
Basically, the word versions are what people would normally use, they are easier
to type but in particular are more explicit on meaning, where your overloaded
symbolics mostly are not; the Unicode are for people who want a concise mathy
look and have a Perl version supporting Unicode operators.
Note that the addition and union only differ for multisets; for sets they are
the same operation.
-- Darren Duncan
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