条件JOIN不同的表

Question

我想知道用户是否在2个相关表中的任何一个中都有一个条目.

表

USER (user_id)
EMPLOYEE (id, user_id)
STUDENT (id, user_id)

用户可以拥有员工和/或学生条目.如何在一个查询中获取该信息？我试过了:

select * from [user] u
inner join employee e 
    on e.user_id = case when e.user_id is not NULL 
                        then u.user_id 
                        else null 
                   end
inner join student s 
    on s.user_id = case when s.user_id is not NULL 
                        then u.user_id 
                        else null 
                   end

但它只会返回两个表中都有条目的用户.

SQL小提琴示例

Answer 1

您可以使用外部联接:

select *
  from USER u
  left outer join EMPLOYEE e ON u.user_id = e.user_id
  left outer join STUDENT s ON u.user_id = s.user_id
 where s.user_id is not null or e.user_id is not null

或者(如果您对EMPLOYEE或STUDENT表中的数据不感兴趣)

select *
  from USER u
 where exists (select 1 from EMPLOYEE e where e.user_id = u.user_id)
    or exists (select 1 from STUDENT s  where s.user_id = u.user_id)

Answer 2

如果您想要将所有用户数据放在一起您可能拥有:

SELECT 
    user_id
    ,'Employee' AS Source
FROM 
    employee
UNION 
SELECT 
    user_id
    ,'Student' AS Source
FROM 
    student

http://sqlfiddle.com/#!3/90216/22

这也可以使用完整连接和CASE语句来完成:

SELECT 
    ISNULL(e.user_id,s.user_id) AS user_id
    ,CASE WHEN e.user_id IS NULL THEN 'Student' 
        ELSE 'Employee'
    END AS SOURCE
FROM 
    employee AS e 
    FULL JOIN student AS s
        ON s.user_id = e.user_id

http://sqlfiddle.com/#!3/90216/29

后者将把同时兼顾员工的人员​​组合成一排,并称他们为员工.相比:

http://sqlfiddle.com/#!3/2aa3e/1 和 http://sqlfiddle.com/#!3/2aa3e/2

我已经让用户1成为学生和员工

Answer 3

如果将员工表和学生表视为一个表，则可以使用左连接：

select * 
from user u
left join 
(
   select 'Employee' as UserType,
          id,
          user_id
     from employee e 
    union all
   select 'Student',
          id,
          user_id
     from student s 
) r
  ON u.user_id = r.user_id

Answer 4

这样的解决方案也可以为您提供帮助。

SELECT  S.*, P.*

,CASE
    WHEN S.ShipmentType = 'import' THEN SP.SupplierName
    WHEN S.ShipmentType = 'export' THEN C.CustomerName
END AS ShipmentDesination

FROM            tblShippments   S 
INNER JOIN      tblProducts     P   ON S.productId = P.productID  
LEFT OUTER JOIN tblCustomers    C   ON S.companyId = C.customerId AND S.ShipmentType = 'export'
LEFT OUTER JOIN tblSuppliers    SP  ON S.companyId = SP.supplierId AND S.ShipmentType = 'import'