每年独特冰淇淋口味的数量

Question

这里有一张不同年份不同口味冰淇淋的桌子（名称：ice_cream_table）：

Year      Flavor
2008         Mint
2008         Mint
2008 Cookie Dough
2008 Cookie Dough
2008    Pistachio
2013    Chocolate
2013 Cookie Dough
2013    Pistachio
2013    Chocolate
2013    Pistachio
2017    Chocolate
2017      Vanilla
2017    Chocolate
2017 Cookie Dough
2017   Strawberry
2019        Mango
2019       Lemon
2019      Vanilla
2019        Mango
2019        Mango
2022    Chocolate
2022    Chocolate
2022         Mint
2022   Strawberry
2022       Cherry

每年我都想知道：有多少种口味是第一次出现，有多少种口味是往年出现的？

这是我的代码：

with yearly_flavor as (
    select 
        year,
        flavor,
        row_number() over (partition by flavor order by year) as rn 
    from 
        ice_cream_table 
    group by 
        flavor, year
),

new_flavor as (
    select
        year, 
        count(flavor) as new_flavor 
    from 
        yearly_flavor
    where 
        rn = 1
    group by 
        year
),

repeated_flavor as (
    select
        year, 
        case 
            when count(flavor) is null then 0 
            else count(flavor) 
        end as repeated_flavor,
        count(flavor) as new_flavor 
    from 
        yearly_flavor 
    where 
        rn > 1
    group by 
        year
),

total_flavor as (
    select
        year, 
        count(distinct flavor) as total_flavor 
    from 
        ice_cream_table
    group by 
        year
)

select 
    n.year,
    n.new_flavor,
    r.repeated_flavor,
    t.total_flavor
from 
    new_flavor n
left join 
    repeated_flavor r on n.year = r.year
join 
    total_flavor t on n.year = t.year
order by
    n.year;

我做得正确吗？我对 Python 更有经验，对 SQL 经验较少……代码可以运行，但我不确定我的逻辑是否抓住了本质。

Answer 1

您可以按如下方式使用

GROUP BY

和

LEFT JOIN

：

select t.year, 
       count(distinct t.flavor) - count(distinct tt.flavor) as new_flavor
       count(distinct tt.flavor) as repeated_flavor,
       count(distinct t.flavor) as total_flavor
  from ice_cream_table t
  left join ice_cream_table tt on t.year > tt.year and t.flavor = tt.flavor
group by t.year