sum

df.sum() // sum of values per every numeric column
df.sum { age and weight } // sum of all values in `age` and `weight`
df.sumFor { age and weight } // sum of values per `age` and `weight` separately
df.sumOf { (weight ?: 0) / age } // sum of expression evaluated for every row

df.age.sum()
df.groupBy { city }.sum()
df.pivot { city }.sum()
df.pivot { city }.groupBy { name.lastName }.sum()