SQL how to handle multiple value join - sql

Question

I need to join assignments and expatriates tables by a combination of ID, effective_start_date and effective_end_date.
I need to get data about employees who have gone to another country during their assignment effective_start_date and effective_end_date. But there is a need to handle cases when during one assignment there have been entered data about employees going to two or more countries - I need to show only one - the last one or the active one (if there is).
In the results I'm getting multiple values for 123 person ID and it's because there are incorrect values entered in assignments table - I need to only show only one of this person 123 date - the information about him going to china (the active one).
So basically, if during one assignment (between effective_start_date and effective_end_date) there is information about him going to 2 different countries, I need to only show one case. I need to correct my select statement so it handles this case somehow.
Edit : This also needs to work when the 2 cases about employee going to another country are historical so I dont think this can be done with sysdate.
Edit nr.2 - updated sql fiddle. I need to show BOTH expatriations for person 321 and ony one for person 123 - this is basically my main goal.
Edit nr.3 - still havent found the solution.
LINK TO SQLFIDDLE
select
ass.person_id,
ass.effective_start_date,
ass.effective_end_date,
exp.date_from,
exp.date_to,
exp.home_country,
exp.host_country
from expatriates exp, assignments ass
where
exp.person_id=ass.person_id
and exp.date_to >= ass.effective_start_date
and exp.date_to <= ass.effective_end_date

Answer 1

As #PuneetPandey already wrote your logic will not catch all overlapping periods.
To get only one row you can use ROW_NUMBER, e.g.
select *
from
(
select
ass.person_id,
ass.effective_start_date,
ass.effective_end_date,
exp.date_from,
exp.date_to,
exp.home_country,
exp.host_country,
row_number()
over (partition by ass.person_id, ass.effective_start_date
order by exp.date_from) as rn
from expatriates exp, assignments ass
where
exp.person_id=ass.person_id
and exp.date_to >= ass.effective_start_date
and exp.date_to <= ass.effective_end_date
) dt
where rn = 1

Answer 2

First of all, I think the query needs to be changed -
and exp.date_to <= ass.effective_end_date to and exp.date_from <= ass.effective_end_date.
Now, if you want any of the visited country, you can select distinct record by personid as below -
select
distinct ass.person_id,
ass.effective_start_date,
ass.effective_end_date,
exp.date_from,
exp.date_to,
exp.home_country,
exp.host_country
from expatriates exp, assignments ass
where
exp.person_id=ass.person_id
and exp.date_to >= ass.effective_start_date
and exp.date_from <= ass.effective_end_date
or, if you want a particular row, you can probably maintain another column for status and have that as '1' if the visit is active else keep that as '0' and use below query -
select
ass.person_id,
ass.effective_start_date,
ass.effective_end_date,
exp.date_from,
exp.date_to,
exp.home_country,
exp.host_country
from expatriates exp, assignments ass
where
exp.person_id=ass.person_id
and exp.date_to >= ass.effective_start_date
and exp.date_from <= ass.effective_end_date
and exp.status = 1

Answer 3

I think you need to join a third table which will a derived table like "X" below:
select
ass.person_id,
ass.effective_start_date,
ass.effective_end_date,
exp.date_from,
exp.date_to,
exp.home_country,
exp.host_country
from expatriates exp, assignments ass, (
SELECT e.person_id, MAX(e.date_from) md
FROM expatriates e
INNER JOIN assignments a ON e.person_id=a.person_id
and e.date_to >= a.effective_start_date
and e.date_to <= a.effective_end_date GROUP BY e.person_id) X
where exp.person_id= X.person_id
and exp.date_from= X.md

Answer 4

Im assuming if a person get fired effective_end_date will be updated and no more expatriates record will appear. So I just select the last date_to in expatriates. That is why I dont see why you need compare date ranges and remove that part from my where.
SQL FIDDLE DEMO
active_or_last_ass AS (
SELECT exp.person_id, date_from, max(exp.date_to) max_date
FROM expatriates exp
WHERE exp.date_from < sysdate
GROUP BY exp.person_id, date_from
)
select
ass.person_id,
ass.effective_start_date,
ass.effective_end_date,
exp.date_from,
exp.date_to,
exp.home_country,
exp.host_country
from
active_or_last_ass ala
inner join expatriates exp
on exp.person_id = ala.person_id
and exp.date_to = ala.max_date
inner join assignments ass
on exp.person_id = ass.person_id