How to query data from one of two tables based on availablity

Question

I have a query which joins multiple tables. For eg.

Select u.username,r.role,s.salary
from user u, roles r, salary s
where u.userid = s.userid
and r.roleid = u.roleid.

Now I need to append the condition to get the data only if the eligibility flag is true, which can be present in two tables (default eligibility and user eligility) . If the data is in user_eligiblity i need to check from user_eligility else i need to get from default eligibility.

Select u.username,r.role,s.salary
from user u, roles r, salary s, default_eligility de, user_eligiblity ue
where u.userid = s.userid
and r.roleid = u.roleid
and (if ue has data check if ue.eligible=y and ue.userid = u.userid 
     else get data from de.eligibity='y' and de.roleid = u.roleid)

User Eligibility may or may not have have rows for the user. If it does not have have to take it from default table.

Answer 1

The join replace your where condition

And the final left join is the one you want to find which user have a match in user_eligiblity the one who doesnt will get null

COALESCE will return ue.eligible if <> null or de.eligible otherwise

Select u.username, r.role, s.salary
from user u 
join roles r 
  on u.roleid = r.roleid
join salary s
  on u.userid = s.userid
join default_eligility de
  on u.roleid = de.roleid 
left join user_eligiblity ue
  on u.userid = ue.userid
WHERE
     COALESCE(ue.eligible, de.eligible) = 'y'