Beginner at R and getting a different output, need to understand why am getting such an output. Please help!

I was trying to find the price per square feet and used dplyr but I got an output with NA

history <- dbReadTable(con,"NYC_HISTORICAL")
building <- dbReadTable(con,"BUILDING_CLASS")
neighboorhood <- dbReadTable(con,"NEIGHBORHOOD")

combine1 <-
history %>%
left_join (y=building, by=c("BUILDING_CLASS_FINAL_ROLL"="X.BUILDING_CODE_ID"))

filter <-
combine1 %>%
filter(TYPE=="RESIDENTIAL")

combine2<-
history %>%
left_join(y=neighboorhood, by= "NEIGHBORHOOD_ID" )

filter3<-
combine2%>%
filter(NEIGHBORHOOD_NAME=="ASTORIA")%>%
group_by(SALE_DATE)

table5<-
filter3 %>%
select(ZIP_CODE,SALE_PRICE,GROSS_SQUARE_FEET) %>%
group_by (ZIP_CODE)

gross1<-
table5%>%
filter(GROSS_SQUARE_FEET>0)
gross1

gross<-summarise(table5, mean(SALE_PRICE/GROSS_SQUARE_FEET))
gross

1 " 0" NaN
2 "10128" Inf
3 "11001" NaN
4 "11006" Inf
5 "11101" NaN
6 "11102" NA
7 "11103" NA
8 "11104" 73.8
9 "11105" NaN
10 "11106" NA
11 "11361" 257.
12 "11363" NaN
13 "11370" NaN
14 "11375" Inf
15 "11377" Inf
16 "11422" Inf

That's my output, can anyone help me why am I getting NA or NAN, and if you explain what it means?

Thank you so much for your help!

Inf means infinity, NaN means not a number and NA means a missing value and, not visible in the output but closely related NULL means undefined.

There's no guarantee that these will go away after SALE_PRICE is converted from class character to class numeric, but that would be a good start.

@technocrat Thanks a lot for your help!