问题 I would like to match multiple conditions from independent data tables onto my main data table. How can I do this using the data.table package? What would be the most efficient/fastest way? I have a mock example, with some mock conditions here to illustrate my question: main_data <- data.frame( pnum = c(1,2,3,4,5,6,7,8,9,10), age = c(24,35,43,34,55,24,36,43,34,54), gender = c("f","m","f","f","m","f","m","f","f","m")) data_1 <- data.frame( pnum = c(1,4,5,8,9), value_data_1 = c(1, 2, 1, 1, 1),