Take Correlated Sub-query Out of CASE WHEN Statement

Question

I have an employee table in Oracle that can have either 1 or 2 "future" jobs in table future_jobs, a business rule of some sort, e.g.

| employee_id | job_id | job_start_date | job_end_date |
|-------------|--------|----------------|--------------|
| 1           | 127589 | 12-SEP-2016    | 25-DEC-2016  |
| 1           | 834780 | 26-DEC-2016    | 08-AUG-2017  |
| 2           | 800253 | 20-OCT-2016    | 13-APR-2017  |

I have to get the description of each future job by invoking a stored procedure with a specific parameter, e.g. F1 and F2, based on a descending order of job_start_date. In example above, for employee_id = 1, when the query below is executed for job_id = 127589 row, since job_start_date = 12-SEP-2016 is the earliest date among the two rows for employee_id = 1, get_description(emp.employee_id, 'F1') should be called, and get_description(emp.employee_id, 'F2') for job_id = 834780.

and for employee_id = 2, since there is only one future job, get_description(emp.employee_id, 'F1') should be invoked with the query below. Currently, I can pull the relevant information with the following query:

select
    emp.employee_id,
    case
        when fj.job_start_date = (select max(job_start_date)
                                  from future_jobs
                                  where employee_id = fj.employee_id
                                  group by employee_id
                                  having count(employee_id) > 1)
        then get_description(emp.employee_id, 'F2')
        else get_description(emp.employee_id, 'F1')
    end job_description,
    fj.job_start_date
    jd.some_additional_columns
from employees emp
join future_jobs fj
    on emp.employee_id = fj.employee_id
join job_details jd
    on  jd.job_id = fj.job_id
    and jd.job_start_date = fj.job_start_date
    and jd.job_end_date = fj.job_end_date

.

| employee_id |    job_description   | job_start_date |  jd.columns  |
|-------------|----------------------|----------------|--------------|
| 1           | 1st future job desc  | 12-SEP-2016    | ....         | 
| 1           | 2nd future job desc  | 26-DEC-2016    | ....         |  
| 2           | 1st future job desc  | 20-OCT-2016    | ....         | 

However, I would like to know if there is another way to take the correlated sub-query out of CASE WHEN statement? Is there even a way to do it without using the correlated sub-query? I need to have this done in a single statement rather than using WITH clause type solution.

Answer 1

I think you just want window functions:

select emp.employee_id,
       (case when fj.seqnum = 1
             then get_description(emp.employee_id, 'F1')
             else get_description(emp.employee_id, 'F2')
        end) as job_description,
       jd.some_additional_columns
from employees emp join
     (select fj.*,
             row_number() over (partition by employee_id order by fj.job_start_date) as seqnum
      from future_jobs fj
     ) fj
    on emp.employee_id = fj.employee_id join
    job_details jd
    on jd.job_id = fj.job_id and
       jd.job_start_date = fj.job_start_date and
       jd.job_end_date = fj.job_end_date;

I'm not 100% sure the logic is exactly correct. It follows your description and uses F1 for the first future job.