Bagaimana cara mengelompokkan berdasarkan bulan dan tahun ketika hanya memiliki bidang datetime?

Anda perlu mengelompokkan berdasarkan ekstrak.

SELECT 
    SUM(amount)
FROM 
    transaction
GROUP BY 
    EXTRACT(MONTH FROM when),
    EXTRACT(YEAR FROM when)

Dan jika Anda membutuhkan kolom tersebut, maka

SELECT
    EXTRACT(MONTH FROM when) as month, 
    EXTRACT(YEAR FROM when) as year, 
    SUM(amount)
FROM 
    transaction
GROUP BY 
    month,
    year

Tentu saja Anda dapat menambahkan ORDER BY dan gunakan nama pendek juga:

SELECT 
    EXTRACT(MONTH FROM when) as month, 
    EXTRACT(YEAR FROM when) as year, 
    SUM(amount)
FROM 
    transaction
GROUP BY 
    month, 
    year
ORDER BY 
    year DESC, 
    month DESC