sql验证两个表之间的关系

Question

I have two tables: A and B

Table A columns:
content_key data_key

Table B columns: content_key person_key

I need to verify that for each data_key there is only one person_key that is associated with it, it can have the same person_key multiple times but only one unique value.

Answer 1

You should be able to use COUNT() and GROUP BY the data_key and person_key

SELECT count(data_key) cnt, data_key, person_key
FROM tableA a
INNER JOIN tableB b
    ON a.content_key = b.content_key
GROUP BY data_key, person_key

Answer 2

A good way to do this is with COUNT(DISTINCT):

select A.data_key, count(distinct person_key) as numPersons
from A join
     B
     on A.content_key = B.content_key
group by A.data_key
order by 2 desc

I added the order by, so you can see the duplicates first.

Well, since you want to verify rather than see the duplicates, add a HAVING clause:

having count(distinct person_key) > 1

If the query returns no rows, then the data meets your condition.

Also, this assumes that person_key is never NULL. The count is a bit more complicated if that can happen:

(COUNT(distinct person_key) + max(case when person_key is null then 1 else 0 end))