MySQL - GROUP BY not working with multiple CASE WHEN statements
A syntactical version that should work (untested due to lack of ddl and sample data) is:
SELECT 
CASE 
    WHEN DATEDIFF(campaign.deadline,campaign.launched)&lt;=30 THEN 'Short Campaign' 
    WHEN DATEDIFF(campaign.deadline,campaign.launched)&gt;30 AND DATEDIFF(campaign.deadline,campaign.launched)&lt;=60 THEN 'Mid-lengthed Campaign'
    ELSE 'Long Campaign'
    END AS 'Campaign Length',
CASE 
    WHEN currency.name='GBP' THEN ROUND(SUM(campaign.pledged)*0.80,2)
    WHEN currency.name='CAD' THEN ROUND(SUM(campaign.pledged)*1.36)
    WHEN currency.name='AUD' THEN ROUND(SUM(campaign.pledged)*1.43)
    WHEN currency.name='NOK' THEN ROUND(SUM(campaign.pledged)*9.28)
    WHEN currency.name='EUR' THEN ROUND(SUM(campaign.pledged)*0.87)
    WHEN currency.name='MXN' THEN ROUND(SUM(campaign.pledged)*22.48)
    WHEN currency.name='SEK' THEN ROUND(SUM(campaign.pledged)*0.04)
    WHEN currency.name='NZD' THEN ROUND(SUM(campaign.pledged)*1.53)
    WHEN currency.name='CHF' THEN ROUND(SUM(campaign.pledged)*0.94)
    WHEN currency.name='DKK' THEN ROUND(SUM(campaign.pledged)*6.52)
    WHEN currency.name='HKD' THEN ROUND(SUM(campaign.pledged)*7.75)
    WHEN currency.name='SGD' THEN ROUND(SUM(campaign.pledged)*1.39)
    WHEN currency.name='JPY' THEN ROUND(SUM(campaign.pledged)*107.11)
    ELSE ROUND(SUM(campaign.pledged),2)
END AS 'Amount Raised'
FROM campaign
LEFT JOIN currency ON currency.id=campaign.currency_id
GROUP BY CASE 
    WHEN DATEDIFF(campaign.deadline,campaign.launched)&lt;=30 THEN 'Short Campaign' 
    WHEN DATEDIFF(campaign.deadline,campaign.launched)&gt;30 AND DATEDIFF(campaign.deadline,campaign.launched)&lt;=60 THEN 'Mid-lengthed Campaign'
    ELSE 'Long Campaign'
    END, currency.name;

I agree with @Akina that the result will be confusing when several currencies are involved. The result may end up like:
Long Campaign   42
Long Campaign   36
...

and you basically have no idea what row that represents which currency