  RDF data in the linked open data (LOD) cloud is very valuable for many
different applications. In order to unlock the full value of this data, users
should be able to issue complex queries on the RDF datasets in the LOD cloud.
SPARQL can express such complex queries, but constructing SPARQL queries can be
a challenge to users since it requires knowing the structure and vocabulary of
the datasets being queried. In this paper, we introduce Sapphire, a tool that
helps users write syntactically and semantically correct SPARQL queries without
prior knowledge of the queried datasets. Sapphire interactively helps the user
while typing the query by providing auto-complete suggestions based on the
queried data. After a query is issued, Sapphire provides suggestions on ways to
change the query to better match the needs of the user. We evaluated Sapphire
based on performance experiments and a user study and showed it to be superior
to competing approaches.
