case when语句第一种方式：
case
when 表达式1 then 结果1
when 表达式2 then 结果2
else 结果n
end
举例1：
select sum(
case
when rental_rate=0.99 then 1
else 0
end
) as "aa",
sum(
case
when rental_rate=2.99 then 1
else 0
end
) as "bb",
sum(
case
when rental_rate=4.99 then 1
else 0
end
) as "cc"
from film;
结果：
aa bb cc
341 323 336
【注】：as后接的别名需要带双引号，否则报语法错误
case when语句第二种方式：
case 表达式
when 匹配1 then 结果1
when 匹配2 then 结果2
else 结果n
end
举例2：
select sum(
case rental_rate
when 0.99 then 1
else 0
end
) as "aa",
sum(
case rental_rate
when 2.99 then 1
else 0
end
) as "bb",
sum(
case rental_rate
when 4.99 then 1
else 0
end
) as "cc"
from film;
结果：
aa bb cc
341 323 336
film源表.png
