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Thanks so much. I can try Jena api for OPTIONAL.
Since Jena api is implemented on top of SEM_MATCH api and supports OPTIONAL, does that mean there is a certain way to use SEM_MATCH to accomplish OPTIONAL?
Jena Adaptor's support of OPTIONAL, UNION, named graph, etc. is done through Jena's ARQ.
Basically, ARQ reads in a query and breaks it apart into multiple requests against the underlying
GraphOracleSem instance. GraphOracleSem will then convert the request into either a SEM_MATCH
or a direct SQL query for some cases.
Your reasoning is indeed making sense. However, SEM_MATCH itself, as of now, does not support
OPTIONAL, UNION, etc.
If you want to achieve OPTIONAL in a SQL-based approach, it is certainly doable though.
You can use multiple SEM_MATCH's and outer joins. Some users have tried this approach.