If I try to get an average of c(NA, NA, 3, 4, 5, 6, 7, 8, 9, 10) using AVG from SQL, I get a value of 5.2, instead of the expected 6.5.
# prepare data and write to file
write.table(data.frame(col1 = c(NA, NA, 3:10)),
"my.na.txt", row.names = FALSE)
mean(c(NA, NA, 3:10), na.rm = TRUE) # 6.5
my.na <- read.csv.sql("my.na.txt", sep = " ",
sql = "SELECT AVG(col1) FROM file") # 5.2
# this is identical to
sum(3:10)/10
unlink("my.na.txt") # remove file
Which leads me to believe that sql(df) treats NA values as zero. Is it possible to ignore (exclude) NA values in an SQL call as it can be done using na.rm argument (in R)?
Answer
Modify your query to ignore the NA values:
SELECT AVG(col1)
FROM file
WHERE col1 IS NOT \"NA\"