How to combine if statements within a single SQL statement (MySQL)?

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SQL

hi everyone,

I´m stuck on a SQL statement which contains the following calculation:

I have three tables in my database foto
1.rates (rate_id, price, frequency),2.rate_changes (old_rate_id, new_rate_id, photographer_id) and payments (created, amount_net)

Further, I want to calculate the amount_net per month depending on the rate frequency but I dn´t know exactly how to do it:

SELECT sum(
if foto.rates.frequency = 1 then (sum foto.payments.amount_net(month x))
elseif foto.rates.frequency = 3 then (foto.payments.amount_net (month x)/foto.rates.frequency) And foto.payments.amount_net (month x-2)+foto.payments.amount_net (month x)/foto.rates.frequency, foto.payments.amount_net (month x-1) + foto.payments.amount_net (month x)/foto.rates.frequency

elseif foto.rates.frequency = 6 then..(the same calculation as in the case before)
elseif foto.rates.frequency = 12 then..(the same calculation as in the cases before))

End

Thanks a lot in advance for your help!