使用GROUP BY时，计算2列的不同值

Question

Considering the following query:

SELECT t.recording_id, m.release_id
FROM track t
JOIN medium m ON t.medium_id = m.medium_id

i get a result set similar to this one

recording id    release id
----------------------------------
1               25
1               25
1               37
1               76
1               300
1               336
2               37
...             ...

i need to output the following

recording id    count
---------------------------------------------------
1              5
2              1

In other words, i need to group by the recording_id but not count the release_id duplicates for that recording_id

After researching this board i've tried the following, with no success :

SELECT t.recording_id, count(t.recording_id)
FROM track t
JOIN medium m ON t.medium_id = m.medium_id
group by t.recording_id, m.release_id

but, im getting

recording id    release id
--------------------------
1              2
1              1
1              1
1              1
1              1
2              1

What's wrong?

Answer 1

Try this, you can use distinct in your count function to return distinct release ids for a recording_id

SELECT t.recording_id, count(distinct m.release_id) cnt
FROM track t
JOIN medium m ON t.medium_id = m.medium_id
group by t.recording_id