DataFrame, aggregate by month of date

General Usage
3

I think the best way is to use Query.jl

using DataFrames, Query

df = DataFrame(name=["John", "Sally", "Kirk", "Sanders"], age=[23., 42., 59., 31.], children=[3,2,2,0], date=[Date("2015-01-01"),Date("2015-01-10"),Date("2015-02-20"), Date("2015-02-05")])

x = df |>
    @groupby(Dates.format(_.date, "yyyy-mm")) |>
    @map({Key=key(_), Count=length(_)}) |>
    DataFrame

println(x)

2×2 DataFrame
│ Row │ Key     │ Count │
│     │ String  │ Int64 │
├─────┼─────────┼───────┤
│ 1   │ 2015-01 │ 2     │
│ 2   │ 2015-02 │ 2     │