A dense index stores an index entry for every search key value in the file, whereas a sparse index stores an entry for only some search key values.   

A sparse index stores an entry for every record in the file, while a dense index stores only one entry per block.  

Dense indexes are used exclusively for non-clustered indexes, while sparse indexes are used exclusively for clustering indexes.

Sparse indexes are unordered, whereas dense indexes are always maintained in sorted order.  

When there are frequent updates to the indexed attribute that require every record to be re-indexed.  

When every record must be directly accessible through the index.  

When the file is sorted on the search key and only one index entry per block is sufficient for retrieval. 

When the file is unsorted and records are randomly stored.  

A dense index stores an index entry for every search key value in the file, whereas a sparse index stores an entry for only some search key values.   

A sparse index stores an entry for every record in the file, while a dense index stores only one entry per block.  

Dense indexes are used exclusively for non-clustered indexes, while sparse indexes are used exclusively for clustering indexes.

Sparse indexes are unordered, whereas dense indexes are always maintained in sorted order.