Sql查询获取名称列为null

Question

Sql小提琴

CREATE TABLE [Users_Reg]
(
   [User_ID]        [int] IDENTITY (1, 1) NOT NULL CONSTRAINT User_Reg_P_KEY PRIMARY KEY,
   [Name]           [varchar] (50) NOT NULL, 
   [Type]           [varchar] (50) NOT NULL /*Technician/Radiologist*/
)

CREATE Table [Study]
(
  [UID]             [INT] IDENTITY (1,1) NOT NULL CONSTRAINT Patient_Study_P_KEY PRIMARY KEY,
  [Radiologist]     [int], /*user id of Radiologist type*/
  [Technician]      [int], /*user id of Technician type*/

)


select * from Study
inner join Users_Reg
on Users_Reg.User_ID=Study.Radiologist

在patient_study表中,放射科医师或技师可能有0值.我如何从查询中获得技术员姓名和放射科医生姓名.

Answer 1

您需要users_reg两次在表上加入以获得结果.一次是放射科医师,另一次是技术人员:

select ps.uid,
  ur1.name rad_name,
  ur1.type rad_type,
  ur2.name tech_name,
  ur2.type tech_type
from Patient_Study ps
left join Users_Reg ur1
  on ur1.User_ID=ps.Radiologist
left join Users_Reg ur2
  on ur2.User_ID=ps.Technician;

请参阅SQL Fiddle with Demo.这将返回所有患者研究的放射科医师和技师名称/类型.如果要替换任何列中的null,则可以使用COALESCE类似于以下内容:

select ps.uid,
  coalesce(ur1.name, '') rad_name,
  coalesce(ur1.type, '') rad_type,
  coalesce(ur2.name, '') tech_name,
  coalesce(ur2.type, '') tech_type
from Patient_Study ps
left join Users_Reg ur1
  on ur1.User_ID=ps.Radiologist
left join Users_Reg ur2
  on ur2.User_ID=ps.Technician;

请参阅SQL Fiddle with Demo