Creates a data.table to be passed in as the i to a [.data.table join.
Each argument is a vector. Generally each vector is the same length but if they are not then usual silent repitition is applied.
Internal argument. SJ sets this to TRUE
J : the same result as calling data.table. J is a direct alias for data.table but results in clearer more readable code.
SJ : (S)orted (J)oin. The same value as J() but additionally setkey() is called on all the columns in the order they were passed in to SJ. For efficiency.
CJ : (C)ross (J)oin. A data.table is formed from the cross product of the vectors. For example, 10 ids, and 100 dates, CJ returns a 1000 row table containing all the dates for all the ids.
J, SJ and CJ are convenience functions for creating a data.table in the context of a data.table 'query' on x.
x[data.table(id)] is the same as x[J(id)] but the latter is more readable.
x must have a key when passing in a join table as the i. See [.data.table