Julia DataFrames.jl - filter data with NA's (NAException)

What's your desired behavior here? If you want to do selections like this you can make the condition (not a NAN) AND (equal to 4). If the first test fails then the second one never happens.

using DataFrames
a = @data([1, 2, 3, 4, 5]);
b = @data([3, 4, 5, 6, NA]);
ndf = DataFrame(a=a, b=b)
ndf[(!isna(ndf[:b]))&(ndf[:b].==4),:]

In some cases you might just want to drop all rows with NAs in certain columns

ndf = ndf[!isna(ndf[:b]),:]