Clustered index is dense or sparse? I am confused about clustered index is dense or sparse. I searched for that, sources saying it is sparse, but can be dense also. What is it exactly?
Clustering index is applied when records are physically ordered on a nonkey field(called clustering field). As clustering field is nonkey so there can be more than one distinct record for a single clustering field value.

The strategy of clustering index is: There is one entry in the clustering index for each distinct value in the clustering field, containing the value, and a pointer to the first block in the data file that has a record with that value for its clustering field.

So clearly it is a sparse index.