看完題目後，我一般先看結果的樣子
從結果可以看出，是按照成績自定義分組
自定義分組用到 group by case when
所以先得出

select
case
when duration/60<5
then '[0-5>'
when duration/60<10
then '[5-10>'
when duration/60<15
then '[10-15>'
else '15 or more'
end as bin,
count(1) as total
from `Sessions`
group by bin

得出的結果跟答案不一樣
因為對於不在判斷條件內的值是不會產生記錄的
比如答案中的 [10-15> | 0

select '[0-5>' as bin union
select '[5-10>' as bin union
select '[10-15>' as bin union
select '15 or more' as bin

得出這一部分後，將兩部分拼接起來，使用左連線
使用ifnull判斷為空的 total項，並將其設為0

select
s1.bin as bin,
ifnull(s2.total, 0) as total
from
(
    select
 
 '[0-5>' as bin union
    select '[5-10>' as bin union
    select '[10-15>' as bin union
    select '15 or more' as bin
) as s1
left join
(
    select
    case
    when duration/60<5
    then '[0-5>'
    when duration/60<10
    then '[5-10>'
    when duration/60<15
    then '[10-15>'
    else
 
 '15 or more'
    end as bin,
    count(1) as total
    from `Sessions`
    group by bin
) as s2
on s1.bin = s2.bin