I want to get data from 3 tables - sql

Question

I have 3 tables employee, jobs and department as below,
------------------- ---------------- ------------------
| employee | | jobs | | department |
------------------- ---------------- ------------------
| empId | | jobId | | deptId |
| fname | | jobTitle | | deptName |
| lname | | | | managerId |
| jobId | | | | |
| managerId | | | | |
| departmentId | | | | |
------------------- ---------------- -------------------
I want to select all data from employee, the job title through jobId, the deptName through deptId and manager name through managerId and employeeId
SELECT EMPLOYEES.EMPLOYEE_ID, EMPLOYEES.FIRST_NAME, EMPLOYEES.LAST_NAME,
EMPLOYEES.JOB_ID, JOBS.JOB_TITLE AS JOB_TITLE, EMPLOYEES.SALARY,
DEPARTMENTS.DEPARTMENT_ID, DEPARTMENTS.DEPARTMENT_NAME AS DEPARTMENT_NAME
FROM EMPLOYEES
LEFT JOIN JOBS ON EMPLOYEES.JOB_ID = JOBS.JOB_ID
LEFT JOIN DEPARTMENTS ON EMPLOYEES.DEPARTMENT_ID = DEPARTMENTS.DEPARTMENT_ID
what should I do to get the name of the manager using the empId?
or is there any other way to simplify this block of code?

Answer 1

Just Add another LEFT JOIN to the EMPLOYEES table, but give it an alias, say MANAGERS.
SELECT
EMPLOYEES.EMPLOYEE_ID,
EMPLOYEES.FIRST_NAME,
EMPLOYEES.LAST_NAME,
EMPLOYEES.JOB_ID,
JOBS.JOB_TITLE AS JOB_TITLE,
EMPLOYEES.SALARY,
DEPARTMENTS.DEPARTMENT_ID,
DEPARTMENTS.DEPARTMENT_NAME AS DEPARTMENT_NAME,
MANAGERS.FIRST_NAME AS MANAGER_FIRST_NAME,
MANAGERS.LAST_NAME AS MANAGER_LAST_NAME
FROM EMPLOYEES
LEFT JOIN EMPLOYEES MANAGERS
ON EMPLOYEES.MANAGER_ID = MANAGERS.EMPLOYEE_ID
LEFT JOIN JOBS
ON EMPLOYEES.JOB_ID = JOBS.JOB_ID
LEFT JOIN DEPARTMENTS
ON EMPLOYEES.DEPARTMENT_ID = DEPARTMENTS.DEPARTMENT_ID

Answer 2

You can use another left join on employee table to get the manager name.
SELECT (
EMPLOYEES.EMPLOYEE_ID,
EMPLOYEES.FIRST_NAME,
EMPLOYEES.LAST_NAME,
EMPLOYEES.JOB_ID,
JOBS.JOB_TITLE AS JOB_TITLE,
EMPLOYEES.SALARY,
DEPARTMENTS.DEPARTMENT_ID,
DEPARTMENTS.DEPARTMENT_NAME AS DEPARTMENT_NAME,
(MGR.FIRST_NAME + ' ' + MGR.LAST_NAME) AS MANAGER_NAME)
FROM EMPLOYEES
LEFT JOIN JOBS
ON EMPLOYEES.JOB_ID = JOBS.JOB_ID
LEFT JOIN DEPARTMENTS
ON EMPLOYEES.DEPARTMENT_ID = DEPARTMENTS.DEPARTMENT_ID
LEFT JOIN EMPLOYEES MGR
ON EMPLOYEES.MANAGERID = MGR.EMPLOYEE_ID

Answer 3

try using aliases and join back to employee on the manager id.
SELECT
E.EMPLOYEE_ID, E.FIRST_NAME, E.LAST_NAME,
E.JOB_ID, J.JOB_TITLE AS JOB_TITLE, E.SALARY,
D.DEPARTMENT_ID, D.DEPARTMENT_NAME AS DEPARTMENT_NAME,
M.EMPLOYEE_ID as mgr_id, M.FIRST_NAME as mgr_name, M.LAST_NAME as mgr_lname
FROM EMPLOYEES E LEFT JOIN JOBS J ON
E.JOB_ID = J.JOB_ID
LEFT JOIN DEPARTMENTS D ON
E.DEPARTMENT_ID = D.DEPARTMENT_ID
join employees M ON
E.manager_ID = M.employee_ID
The reason this query works is because I am doing a self join back to the Employees table from the main result set that you already established. To put it simply, imagine you made an exact copy of the employees table and called it M. If you took your original query and joined to the M table using the original query's Employee.manager_id to the employee_ID in the M table, then you would have the manager for the employee.
There is no need to create an exact copy of the employee table just to look up the manager. We can just reference the employee table a second time and use an alias (I used M for manager). Then we join from your list of employees using the manager_id to get the employee's manager.
You could do this again to get the manager's manager if you need to. Here is that query:
SELECT
E.EMPLOYEE_ID, E.FIRST_NAME, E.LAST_NAME,
E.JOB_ID, J.JOB_TITLE AS JOB_TITLE, E.SALARY,
D.DEPARTMENT_ID, D.DEPARTMENT_NAME AS DEPARTMENT_NAME,
M.EMPLOYEE_ID as mgr_id, M.FIRST_NAME as mgr_name, M.LAST_NAME as mgr_lname
FROM EMPLOYEES E LEFT JOIN JOBS J ON
E.JOB_ID = J.JOB_ID
LEFT JOIN DEPARTMENTS D ON
E.DEPARTMENT_ID = D.DEPARTMENT_ID
join employees M ON /* The employee's manager */
E.manager_ID = M.employee_ID
LEFT join employees MM ON /* The employee's manager's manager */
M.manager_ID = MM.employee_ID
I used a left join for this last one, because at some point you will get to the top of the management hierarchy and might find that there are no more managers. You could also put a left join on the join employees M
Here it is in Tabular form
Employee_id | Name | manager_id
1 | Fred | 10
2 | Jane | 10
10 | Bob | 20
20 | Betty | Null
Looking at employee # 1. The values of E.employee_id = 1, E.Name = Fred and E.manager_id = 10.
So the relevant lines of the query evaluate as follows:
...
join employees M ON /* The employee's manager */
E.manager_ID /* i.e. 10 */ = M.employee_ID
...
So the M alias now refers to the employee record where M.employee_ID = 10 and as such, M.Name = Bob and M.employee_id = 20.
Using the last version of the query, we could then work out that Fred's manager's manager (i.e. Fred's manager is Bob and Bob's manager is Betty) will have an employee_id of 20 (i.e. M.manager_ID = 20), so the MM.employee_id would be 20 and hence refer to Betty who doesn't seem to have a manager.