同一张表，1个字段到2个字段的查询

Question

我有2个表格：第1个持有员工（任何职位的员工）和第2个持有管理人员与ID号码的员工关系。

我想写像

1st field: name(employee), 
2nd field: name(manager) 

查询我怎样才能做到这一点？

Answer 1

无需嵌套查询，只需使用标准连接：

select e.*, m.* 
from 
    employee e 
    left join employee_managers em 
     on e.id = em.emp_id 
    left join employee m 
     on m.id = em.man_id 

每行将包含employee的所有字段（如果一个员工有几个关联的经理，则可能有多行）以及他对应经理的所有字段（如果员工没有经理，则为NULL）。

Answer 2

你可以做一个表：

Employee 
-------- 
EmployeeId int 
Name varchar(50) 
ManagerId int 

ManagerId指向经理在同一个表项。首席执行官的经理ID将为null。一个例子表定义：

create table Employees (
    EmployeeId int auto_increment primary key 
, Name varchar(50) 
, ManagerId int 
, foreign key (ManagerId) references Employees(EmployeeId) 
); 

一些示例数据：

insert into Employees (Name) select 'The Chief'; 
insert into Employees (Name, ManagerId) select 'Grunt 1', 
    (select EmployeeId from Employees where Name = 'The Chief'); 
insert into Employees (Name, ManagerId) select 'Grunt 2', 
    (select EmployeeId from Employees where Name = 'The Chief'); 
insert into Employees (Name, ManagerId) select 'Secretary', 
    (select EmployeeId from Employees where Name = 'The Chief'); 

找到第二步兵的经理的名字，你可以查询，如：

select mgr.Name 
from Employees mgr 
inner join Employees grunt 
on grunt.managerid = mgr.employeeid 
where grunt.name = 'Grunt 2';