Hi Mark!
On Wed, Nov 20, 2013 at 01:11:11PM -0800, Mark wrote:
> I'm using the 2.0 snapshot. How should I express a join on multiple
> columns, ie
> SELECT blah FROM t1 JOIN t2 ON (t1.a=t2.b) AND (t1.c=t2.d) ?
Something like this should work:
(let [t1 (-> (table :t1)
(project [:a :c]))
t2 (-> (table :t2)
(project [:b :d]))]
(join t1 t2 :on `(and (= :a :b)
(= :c :d))))
If you've got shared attributes this won't work for you, though, because
then there's an ambiguity about whether to use a natural join (ie.
joining on equality of shared attributes) or your join condition (which
will leave an ambiguity about which shared attribute to use). In that
case you can use something like this:
(let [t1 (-> (table :t1)
(project [:a :b]))
t2 (-> (table :t2)
(project [:a :c]))]
(join (rename t1 (as-subobject :t1))
(rename t2 (as-subobject :t2))
:on `(and (= :t1.a :t2.c)
(= :t2.a :t1.b))))
Sorry about the lack of documentation for this. I'm working on it, but
I've been quite busy/distracted lately. Hopefully I'll have some more
time to get back to it soon.
Carlo
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