yes sparse index also sequential
but there every record neednot get an entry
The Line is :" ordering of the data records is the same as or close to the ordering of data entries in some index "
The good example of clustering index is telephone directory
Where the index pages start with say "M". All entries in that pages is with M or N or we can say very close to M.
That is why it is clastering index as ans
(While only book is an example of unclustered index)