sql - Why does this GROUP BY and NA combo produce a character type? -

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why y end character class? seems should never happen sqldf sum?

library(sqldf)  # 3 similar data.frame objects x <- structure(list(size = c(1l, 2l), diff = c(1, na)) , .names = c("gb","diff"), row.names = 1:2, class = "data.frame") y <- structure(list(size = c(1l, 1l, 2l, 2l), diff = c(na, na, 1, na)) , .names = c("gb","diff"), row.names = 1:4, class = "data.frame") z <- structure(list(size = c(2l, 2l, 1l, 1l), diff = c(na, na, 1, na)) , .names = c("gb","diff"), row.names = 1:4, class = "data.frame")   # when summed in sqldf: numeric, character, numeric sapply(sqldf("select sum(diff) x"),class) sapply(sqldf("select sum(diff) , gb y group gb"),class)[1] sapply(sqldf("select sum(diff) , gb z group gb"),class)[1]    # despite both being numeric class( x$diff ) class( y$diff )

exclude na, (i.e.: null) before aggregating:

out1 <- sqldf("select sum(diff) diff_sum                x                diff not null")  out2 <- sqldf("select sum(diff) diff_sum, gb                y                diff not null                group gb")  str(out1) # 'data.frame': 1 obs. of  1 variable: #   $ diff_sum: num 1 str(out2) # 'data.frame': 1 obs. of  2 variables: #   $ diff_sum: num 1 # $ gb      : int 2

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