Question

In a SparePart table the uniqueness of the rows is given from a combination of materialName and materialNumber , what i want to do is get the total number of Spare Parts avalaible now.

I have tried using count(distinct …) but it only works for one field not two.

The SQL query(using hibernate query language) with an inner query would look like this :

select count(*) from (
     select distinct materialName, materialNo from SparePart
) as col

Answer 1

Use a subquery+group by and then filter for the first ID that represents the particular group, finally count all those that match the filter:

SELECT COUNT(sp2) FROM SparePart sp2 WHERE EXISTS
  (SELECT sp.materialName, sp.materialNo
          FROM SparePart sp
          GROUP BY sp.materialName, sp.materialNo
          HAVING MIN(sp.id)=sp2.id)