Seems like from a historical perspective hierarchical databases were more prevalent in the 1960s and relational databases didn't exist until the 1970s. (check references for this historically)

Of course one must consider that within a card index or zettelkasten the ideas of both could have co-existed in essence even if they weren't named as such. Some of the business use cases as early as 1903 (earlier?) would have shown the idea of multiple storage and relational database usage. Beatrice Webb's usage of her notes in a database-like way may have indicated this as well.