So you have TableA with a column X, and TableB with column Y. And you want to make Y a foreign key, such that all values of TableB.Y are values in TableA.X, correct?

To do so, TableA.X needs to have either a primary key or a unique constraint on it. It sounds like that is not the case. Make TableA.X unique first, then define your FK from TableB.Y referencing TableA.X

Finding columns that do not match existing primary key
We see a flashback to when Kagami lived in the US in episode 1 of season 2 (26Q). 



Judging from all the palm trees, I'd hazard a guess that he was probably in California. California also seems like the most likely option on a demographic basis (i.e. where are Japanese temporary immigrants most likely to end up?).

Is it ever explicitly stated where he lived? And if it is California, is it ever stated whether he's in SoCal or NorCal (or somewhere else)?
