select case 
					when category = 1 and supplierId = 1 then ..
					when .. then ..
					else 'etc'
				end as categoryName, * -- 두개의 field 가능 
from products 

select case
					when categoryID = 1 then '음료'
					when categoryID = 2 then 'source'
					else 'etc'
				end as new_category
		, avg(price)
from producdts
group by categoryID

-- case 절의 순서를 주의하자 ex) 정삼각형 -> 이등변 삼각형 -> ... 

table pivot

select 
	avg(case when categoryId = 1 then price else null end) as avgPrice_1
	avg(case when categoryId = 2 then price else null end) as avgPrice_2
from products