How to summarise all factor levels with data.table?

Let’s see a minimal example:

library(data.table)
DT = data.table(x=factor(rep(c("b","a","c"),each=3)))
DT[x!="c"][, .N, by = x]
        x     N
   <fctr> <int>
1:      b     3
2:      a     3

In this example, as x is a factor with 3 levels a,b,c, I would like the output was

        x     N
   <fctr> <int>
1:      b     3
2:      a     3
3:      c     0

or having some argument to control this. Is there some way to get this output?

Thanks!

>Solution :

Using the join syntax

DT[x != "c"][levels(x), on = "x", .N, by = .EACHI]

#         x     N
#    <char> <int>
# 1:      a     3
# 2:      b     3
# 3:      c     0