如何分支多个表

Question

中查询，如果我有4个表是这样的：

1- mainemployees 

emp_id 
email 
type (0-->ext,1-->internal,2--->special) 

---

2-externalemp 

emp_id 
name 

3-internalemp 

emp_id 
name 

4-specialemp 

emp_id 
name 

---

现在我希望所有的employee names ，其中第一表的emp_id =最后三张表 之一的emp_id如何做到这一点（性能明智）。

Answer 1

select nvl(ee.name, nvl(ie.name, se.name)) 
from mainemployees me 
left join 
     externalemp ee 
on  ee.emp_id = me.emp_id 
left join 
     internalemp ie 
on  ie.emp_id = me.emp_id 
left join 
     specialem se 
on  se.emp_id = me.emp_id 
where nvl(ee.name, '') <> '' 
     or nvl(ie.name, '') <> '' 
     or nvl(se.name, '') <> '' 

Answer 2

create table mainemployees (emp_id int, emp_type int) 

create table externalemp (emp_id int, name nvarchar(20)) 

create table internalemp (emp_id int, name nvarchar(20)) 

create table specialemp (emp_id int, name nvarchar(20)) 

insert into mainemployees (emp_id, emp_type) values (1, 0), (2, 1), (3, 2) 

insert into externalemp (emp_id, name) values (1, 'external') 
insert into internalemp (emp_id, name) values (2, 'internal') 
insert into specialemp (emp_id, name) values (3, 'special') 

而对于查询使用CASE选择右列

SELECT 
    CASE me.emp_type WHEN 0 THEN ee.name WHEN 1 THEN ie.name WHEN 2 THEN se.name END 
FROM mainemployees as me 
LEFT JOIN externalemp as ee ON me.emp_id = ee.emp_id 
LEFT JOIN internalemp as ie ON me.emp_id = ie.emp_id 
LEFT JOIN specialemp as se ON me.emp_id = se.emp_id