SELECT Id, IF(type = 'P1', amount, amount * -1) as amount
If the condition is null, in the case of a null amount you can use this to handle it:
SELECT Id, IF(type = 'P1', IFNULL(amount,0), IFNULL(amount,0) * -1) as amount
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In the Above code, IFNULL(amount,0) this means it will return amount if the amount is assigned not null but if the amount is assigned as null then it’ll result in ‘zero’.