Annotea. In Annotea, users can access annotations of a document through its URI. Specialized HTTP servers receive the URI in query and return all the RDF statements created for that URI. Client applications are expected to highlight these annotations in a way, and allow users to pick one for more detail. Actual contents of annotations are retrieved if a user chooses one of the annotations highlighted. Client applications can also filter RDF statements if a document is heavily annotated. Annotea does not specify any search mechanism on the annotation servers.
Sesame. The Sesame’s query engine is based on the RDF Query Language (RQL) [69, 4]. RQL is a proposal for a declarative language for RDF and RDF Schema. A RQL query statement is parsed and used to create a query model, which further becomes an optimized query model, where each object performs its own query. Such a model allows queries, hence the results, in Sesame, to be streamed directly to the clients as queries are performed while sacrificing from performance due to the propagating data through abstract layers and the repository.
RDFSuite. RDFSuite also uses RQL as a query language. In oppose to Sesame, such expressions are converted into SQL3  statements and forwarded to the DBMS for performance gains. However, using this approach, RDFSuite becomes more dependent upon DBMSs while preventing users from obtaining advantages of other forms of metadata stores.
SAM. SAM and CMCS use Slide’s database features to query the directory structure in a repository and the metadata associated with each resource, or file. A programming interface translates object queries into corresponding SQL queries, and executes within the relational database. Queries can also be based on associated metadata which Slide stores as attribute-value pairs within database tables.
IKM. IKM provides queries on metadata stored in database tables using SQL statements. Users can perform searches on generic metadata, tribe- or rights-specific information, or access rights. IKM can also aggregate mixed multimedia resource, i.e. images and audio clips, and generate a SMIL (Synchronized Multimedia Integration Language)  presententation through SMIL-capable browsers.