sqldf package in R

# Test some sql  library(sqldf)  id <- c(1,1,1,2,2,1,2) size <- c("big", "small", "bigger", "biggest", "small", "large", "small") df <- data.frame(id, size)  # ======== example: Use of case =============== df1 <- sqldf('              select id, size,                   (case  when size="small" then 1                         when size="big" then 2                         when size="bigger" then 3                         when size="biggest" then 4                         else 0                   end) as size_num              from df              ')  # id    size size_num # 1  1     big        2 # 2  1   small        1 # 3  1  bigger        3 # 4  2 biggest        4 # 5  2   small        1 # 6  1   large        0 # 7  2   small        1  df2 <- sqldf('              select id, size,                   (case  when size="small" then 1                         when size="big" then 2                         when size="bigger" then 3                         when size="biggest" then 4                         else 0                   end) as size_num              from df              order by id ASC, size_num DESC              ')  # id    size size_num # 1  1  bigger        3 # 2  1     big        2 # 3  1   small        1 # 4  1   large        0 # 5  2 biggest        4 # 6  2   small        1 # 7  2   small        1