用Mapping将两个表连接起来并得到结果

Question

I'm use HBase DB with phoenix, I have 3 tables in which two are Main table which have many to many relation ship between them which is mapped by the third table.

Employee

    EmpID    EmpName
    1        Robert
    2        John
    3        Sansa
    4        Ned
    5        Tyrion
    6        George
    7        Daenerys
    8        Arya
    9        Cersie
    10       Catelyn

Department

    DepID    DepName
    1        Hardware
    2        Software
    3        Admin
    4        HR

Department_Employee_Mapping

    ID  DepID   EmpID
    1   1       2
    2   1       6
    3   2       1
    4   3       5
    5   3       6
    6   4       3
    7   4       7
    8   4       10
    9   4       5

I want to get names and department of all the employees who are in Admin Department, but also I want the details of those employee who belong to other department and who do not belong to any of the departments and these data should appear as NULL in the resultset Only the value for admin department should appear and if the employee belong to multiple departments, the result set should contain the value the Admin department and the other one will be ignored, Result set will look like

    Emp Name    Dep Name
    Robert      NULL
    John        NULL
    Sansa       NULL
    Ned         NULL
    Tyrion      Admin
    George      Admin
    Daenerys    NULL
    Arya        NULL
    Cersie      NULL
    Catelyn     Admin

Answer 1

Use 2 LEFT JOIN and include the filtering on 'Admin' in the join against Department

SELECT DISTINCT e.empname, d.depname
FROM Employee e
LEFT JOIN Department_Employee_Mapping x ON x.empid = e.id
LEFT JOIN Departmentd ON d.id = x.depid AND d.id = 3