R，sqldf和avg用于比率平均值

Question

Why do the two commands yield different results when using sqldf in R?

sqldf('select species,
    avg([Petal.Width]/[Petal.Length]) 
    as petalratio from iris group by species')

sqldf('select species,
    ([Petal.Width]/[Petal.Length]) 
    as petalratio from iris group by species')

Being that the goal is to find the average of the ratios for each of the 3 species.

Answer 1

select species,
avg([Petal.Width]/[Petal.Length]) as petalratio 
from iris
group by species

This query outputs the average per species as you are using an aggregate function avg .

select species,
([Petal.Width]/[Petal.Length]) as petalratio 
from iris
group by species

This query randomly outputs one row per species as you aren't using an aggregate function. This isn't allowed in most databases, but allowed in SQLite, which is the default database sqldf uses.

You should use the first query with avg as that is what you are trying to do.