使用SQL进行多对多查询

Question

EmployeeGroups有许多MedicalPlans。 MedicalPlans有许多EmployeeGroups（多对多关系）。它们连接虽然EmployeeBenefits连接表

EmployeeGroup 
:id 
:name 

MedicalPlan 
:id 
:name 

EmployeeBenefits 
:id 
:employee_group_id 
:medical_plan_id 

有谁知道查询我可以用它来获取所有谁有一定MedicalPlan的EmployeeGroups？

我不相信INNER JOIN在这里工作，因为他们没有共同的属性。任何帮助，将不胜感激！

Answer 1

是，内连接工程

select eg.name 
from EmployeeGroup eg 
join EmployeeBenefits eb on eb.employee_group_id = eg.id 
join MedicalPlan mp on eb.medical_plan_id = mp.id 
where mp.name = 'my plan' 

Answer 2

使用EXISTS或IN这样的任务：

select * 
from employeegroup 
where id in 
(
    select employee_group_id 
    from employeebenefits 
    where medical_plan_id = (select id from medicalplan where name = 'XYZ') 
);