R data.table按组统计多列中的唯一值

删除列名周围的引号，以便在传递给

uniqueN

函数之前，列可以作为向量进行计算，否则将作为文字字符向量进行计算：

have[, UnN = uniqueN(c("colA","colB")), by = C("ID")]

have[, UnN = uniqueN(c("colA","colB")), by = C("ID")]

have[, .(UnN = uniqueN(c(colA, colB))), ID]
#   ID UnN
#1:  1   3
#2:  2   3