針對一對多關係的單個SQL SELECT查詢

Question

如果沒有示例，此問題很難解釋。

我有2個表，公司和員工，他們有一個一對多的關係，一個公司可以參加很多員工。

最簡單的表結構如下所示

Company 
| id | name | 

Employee 
| id | name | company_id | join_date | 

現在的問題是：

我怎麼會選擇第2名員工，並展示自己的加盟日期爲公司表列？

所以結果看起來像這樣

| id | company_name | first_employee_join_at | second_employee_join_at | 

Answer 1

假設有在employee表的外鍵列company_id：

with emps as (
    select id, name, company_id, 
     row_number() over (partition by company_id order by join_date) as rn 
    from employee 
) 
select c.id, c.name as company_name, 
     e1.join_date as first_employee_join_at, 
     e2.join_date as second_employee_join_at 
from company c 
    left join emps e1 on e1.company_id = c.id and e1.rn = 1 
    left join emps e2 on e2.company_id = c.id and e2.rn = 2; 

這不會是非常有效的，但。一個稍微更高效的版本將使用條件聚合：

with emps as (
    select id, name, company_id, 
     row_number() over (partition by company_id order by join_date) as rn 
    from employee 
) 
select c.id, c.name as company_name, 
     max(e.join_date) filter (where rn = 1) as first_employee_join_at, 
     max(e.join_date) filter (where rn = 2) as second_employee_join_at 
from company c 
    join emps e on e.company_id = c.id and e.rn in (1,2) 
group by c.id, c.name;