Select minimum data of grouped data - keeping all columns [duplicate]

Answer 1

If you wanted to just use the base functions you can also go with the aggregate and merge functions.

# data (from response above)

df1 <- structure(list(ID = c("A", "A", "A", "B", "B", "C"), c1 = 1:6, 
                  c2 = 1:6, myDate = c("01.01.2015", "02.02.2014", "03.01.2014", 
                                       "09.09.2009", "10.10.2010", "06.06.2011")),
             .Names = c("ID","c1", "c2", "myDate"),
             class = "data.frame", row.names = c(NA,-6L))

# convert your date column to POSIXct object

df1$myDate = as.POSIXct(df1$myDate,format="%d.%m.%Y")

# Use the aggregate function to look for the minimum dates by group. 
# In this case our variable of interest in the myDate column and the
# group to sort by is the "ID" column.
# The function will sort out the minimum date and create a new data frame
# with names "myDate" and "ID"

df2 = aggregate(list(myDate = df1$myDate),list(ID = df1$ID),
            function(x){x[which(x == min(x))]})

df2

# Use the merge function to merge your original data frame with the
# data from the aggregate function

merge(df1,df2)