Trying to enhance the query to give me results - sql

Question

I have 3 three scenarios
where there is no email - returns me empty
where there are multiple emails with one primary email - should return me primary
where there is multiple emails but no primary - should return me the first one
Here is my query I am trying
select *
from departs
left outer join answers on answers.fkdepartid =departs.departID
inner join emails on emails.userid = departs.userID
and emails.primary = 1
where departs.departid = 100
where I can add the logic of the above

Answer 1

You can do this with the row_number() windowing function:
SELECT *
FROM (
SELECT *,
row_number() over (
partition by d.userID
order by case when e.primary = 1 then 0 else 1 end
) rn
FROM departs d
LEFT JOIN answers a on a.fkdepartid = d.departID
INNER JOIN emails e ON e.userid = d.userID
WHERE d.departid = 100
) t
WHERE t.rn = 1
An APPLY lateral join will also work:
SELECT *
FROM departs d
LEFT JOIN answers a on a.fkdepartid = d.departID
CROSS APPLY (
SELECT TOP 1 *
FROM emails e0
WHERE e0.userid = d.userID
ORDER BY case when e0.primary = 1 then 0 else 1 end
) e
WHERE d.departid = 100
APPLY tends to be slower, but I often find it easier to reason about, which can sometimes matter more than the raw performance.
Note both of these options are NON-DETERMINISTIC, which means they could return different results from one moment to the next. This is because there is not a complete definition of "first" for which email to use if none is primary. In database circles, non-deterministic queries and requirements are frowned upon and something to avoid.
To fix this (because it really is broken until you do) to be fully deterministic, add criteria to the ORDER BY clause to further define which email address you want.