Hi All!
I have a problem. When I try craft such URL:
I should get 3 columns (trafficSource, users_left, users_right), but I get only two columns (trafficSource, users). Why?

It looks like you pulled this example from the Metrica API documentation. From their comparison page it seems you would only expect two columns to be returned: one row for each trafficSource.

The Metrica API query runner was a community submission, so we can’t do much to fix it if it breaks. In this case, however, the query runner looks fine. If you have questions about using the Metrica API I’d suggest using their support as this forum isn’t the place for such questions.