一对多关系的单SQL选择查询

没有例子，这个问题很难解释

我有两张表，公司和员工，他们有一对多的关系，一个公司可以接待多个员工

这里显示了最简单的表结构

Company
| id | name |

Employee
| id | name | company_id | join_date |

现在的问题是：

如何选择前两名员工并在公司表中以列形式显示他们的加入日期

结果是这样的

| id |  company_name |  first_employee_join_at |  second_employee_join_at |

---

假设在

employee

表中有一个外键列

company\u 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;

不过，这并不是非常有效。更高效的版本将使用条件聚合：

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;

---

假设在

employee

表中有一个外键列

company\u 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;

不过，这并不是非常有效。更高效的版本将使用条件聚合：

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;

---

你说的前两名员工是什么意思？“前两名员工”我猜是先加入公司的，对吧？你能提供列定义吗？公司和员工是如何联系的？某处有公司id吗？你说的前两名员工是什么意思？“前两名员工”我想是先加入公司的人吧？你能提供列定义吗？公司和员工是如何联系的？某处是否有

公司id

？