Add 'where' argument to [

[MichaelChirico]

This will greatly simplify the code for doing conditional joins/conditional column updates.

Right now there's a collision conflict where i can't be used to do a logical subset and a join simultaneously. Proposed argument would allow this, e.g.:

DT[DT2, on = 'ID', where = V1 < 10, V2 := pmax(i.V3, 0)]

Current version looks quite ugly/unintuitive IMO:

DT[V1 < 10, V2 := DT2[.SD, on = 'ID', pmax(x.V3, 0)]]

[jangorecki]

Adding where argument like this may be seen confusing. In sql where could be used to specify join condition as well. It make sense to postpone your request to a reworked join interface. @arunsrinivasan