Shouldn't the last two commands below yield identical results?
DT <-
data.table(
CIK = c("0000060512", "0000839087"),
FileDate = c(8680, 10081))
CIK = "0000839087"
DT[CIK == (CIK)][FileDate <= 10104]
CIK FileDate
1: 0000839087 10081
DT[CIK == (CIK) & FileDate <= 10104]
CIK FileDate
1: 0000060512 8680
2: 0000839087 10081