Discuss the difference between each of the following types of indexes:

(a) Dense versus sparse indexes.
(b) Primary versus secondary indexes.
(c) Clustered versus unclustered indexes.

(a) A dense index has at least one data entry for every search key value that appears in a record in the indexed file. A sparse index contains an entry for each page of records in a data file. It must be clustered (so we can only have one sparse index on a data file).
(b) A primary index is an index on a set of fields that includes the primary key and is guaranteed not to contain duplicates. A secondary index is not a primary index (and may have duplicates).
(c) A clustered index is one in which the ordering of data entries is the same as the ordering of data records. There can be at most one clustered index on a data file. An unclustered index is an index that is not clustered. There can be more than one unclustered index on a data file.