Q1: Are all DBpedia concepts associated with its relevent wikidata ID?
Those kind of links are usually generated by the people on either side into the direction of the other side. Since it is sometimes very hard to automatically detect links, it is very unlikely that a link between all concepts, properties or entities exist. If you are interested in the linking process, you could check out some research papers which describe how those graph databases generated links to external graphs:
https://scholar.google.de/scholar?q=wikidata+dbpedia+linking
Q2: How to get the wikidata ID associated with DBpedia using sparql?
By using the SPARQL endpoint of dbpedia: http://dbpedia.org/sparql. You have to first select all owl:sameAs
results and then filter them, so you only end up with your Wikidata URI.
Example query:
SELECT distinct ?wikidata_concept
WHERE {dbr:Word2vec owl:sameAs ?wikidata_concept
FILTER(regex(str(?wikidata_concept), "www.wikidata.org" ) )
}
LIMIT 100