national capital vs Riga

national capital and Riga both are nouns.

Word	Noun	Adjective	Verb	Adverb
national capital	Yes	No	No	No
Riga	Yes	No	No	No

national capital (noun)	Riga (noun)
the capital city of a nation	a port city on the Gulf of Riga that is the capital and largest city of Latvia; formerly a member of the Hanseatic League