单SQLSELECT查询一对多关系

Question

这个问题没有例子很难解释。

我有2个table，公司和员工，他们是一对多的关系，一个公司可以参加很多员工。

此处显示最简单的 table 结构

Company
| id | name |

Employee
| id | name | company_id | join_date |

现在的问题是：

我如何 select 前 2 名员工并将他们的入职日期显示为公司 table 的列？

所以结果是这样的

| id |  company_name |  first_employee_join_at |  second_employee_join_at |

Answer 1

假设employee中有一个外键列company_id table:

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;

虽然这不会非常有效。稍微更有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;

单SQLSELECT查询一对多关系

Single SQL SELECT query for one to many relationship

sql

postgresql

one-to-many