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Prolog Variables in CQL queries |
A Prolog variable can be simultaneously a SELECT variable, a JOIN variable and a WHERE variable as A is in the following example:
{[], se_lt_x :: [a-A, c-C] =*= se_lt_y :: [d-A, f-F], A == 'A4'}
which generates the following SQL
SELECT x_192.a, x_192.c, y_73.d, y_73.f FROM se_lt_x x_192 INNER JOIN se_lt_y y_73 ON y_73.d=x_192.a WHERE x_192.a = ? and y_73.d = ?
Note how all the variables referenced in the query are retrieved in the SELECT. This is done to make the query Prolog-like. This means the retrieved row should behave like a Prolog fact so that when a query succeeds all the variables become instantiated.
There is one notable exception however: WHERE variables and JOIN variables are not bound in aggregation selections
FIXME: Is this still the case?
sum_test :- {[], #se_lt_x :: [a-ValueA, sum(b)-Summation] =*= #se_lt_y :: [e-ValueB], ValueA == ValueB, % Explicit join point group_by([ValueA])}, writeln(ValueA-ValueB-Summation).
'ELSTON_M'-_G375971-99450 true ;