[CRConrad]

AIUI, you can have a key-value store is an SQL RDBMS: Just make a big bunch of two-column -- key and value -- tables.

Also, again AIUI, you can build an RDBMS on top of a key-value store: "Just" build a whole lot of key-value pairs where the "value" is a reference to another key more often than not. (Perhaps not just for actual references to other related tables -- or what would be another table in a real RDBMS -- but for each separate key-value store that would just be another column in a real RDBMS. Then I suppose your primary ("key column") key-value pair would have a lot of rows for each key; one for each "column"?)

I suspect most real-world usage of key-value stores in one fashion or another approaches the above... And then the vaunted "simplicity" has gone out the window (and made SPLAT! on the ground far below).

But by then you're so [invested in | married to | sunk cost-fallacied with] your key-value store that changing to a real RDBMS is nigh impossible.

So better start with a real RDBMS from the beginning.