Try this:

DECLARE @period VARCHAR(10)

SET @period = 'current'

select * from t_rbt_data

where

dt >= '2014-01-01' and

dt < '2014-04-01' -- this ensures you get all records within the specified time frame regardless of time

AND toy_type = 'robot'

AND (@period <> 'current' or (@period = 'current' and material = 'metal'))