sql - What is it called when there are two columns in a table that are individually unique? -

as question asks i'm wondering call when row uniquely identified 2 separate columns.

i apologize lack of formatting, here goes:

columna: 1,2,3,4,5

columnb: a,b,c,d,e

columnc: 1,2,1,2,1

columnd: a,a,b,b,a

so both column , column b each individually primary keys? don't think correct, call in terms of "keys"?

if mean every row has unique value , every row has unique b value each of them "superkey". (actually, sets {a} , {b}.)

so answer is, table has 2 one-column superkeys.

but according data no smaller set of columns of either superkey superkey, each "candidate key". (the smaller set {}, it's not superkey given data.)

so here equivalent answer is, table has 2 one-column candidate keys.

a base table , can hold example data in 5nf. there no benefit in decomposing other tables.

there tradition pick candidate key "primary key". each other 1 "alternate key". not particularly helpful tradition.

in sql primary key or unique not null declares superkey. can have 1 primary key declared per table. sql not allow declaring {} superkey.