You didn't specify where the value colR comes from but in general, using Oracle SQL the query could look something like if I understood the question correctly
INSERT INTO Table_Y (colP, colQ, colR)
SELECT a.colA,
a.colB,
(for example some scalar query goes here)
FROM Table_X a
WHERE NOT EXISTS (SELECT 1
FROM Table_Y b
WHERE b.colP = a.ColA)