Question

I am having issues with a sql query.

I have following table (I simplified it a bit):

 ChainId    ComponentId    TransmitEndpointId
   1            156               NULL
   1            156               7
   1            157               7
   2            510               6
   2            510               6
   2            511               6
   2            511               8

What I need to do is get the number of TransmitEndpointId’s foreach ‘unique’ ComponentId in a ‘unique’ ChainId.
So the result of the above data would be: 5 (2 unique componentId’s in Chain 1 & 2 unique componentId’s in Chain 2 with 2 different TransmitId’s ==> NULL values don’t count)

This is quite complex and have no idea on how to start with this query.

Can anybody help?

Thanks in advance.

Answer 1

add the where TransmitEndpointId is not null or you will get a 0 because of the NULL

SELECT ChainId,
       ComponentId,
       count(distinct TransmitEndpointId)
FROM chain
where TransmitEndpointId is not null
GROUP BY ChainId, ComponentId

select sum(endPointNr) from 
(
SELECT count(distinct TransmitEndpointId) as     endPointNr
FROM chain
where TransmitEndpointId is not null
GROUP BY ChainId, ComponentId
) X

EDIT (why add TransmitEndpointId is not null)