Run a single-column SELECT and collect that column's values into a Set of
strings — the shared shape of the "which hashes/names already exist" reads
(e.g. the live table names, the unsubscribed contact hashes).
Parameters
sql: string
column: string
optional
args: InValue[]
Return Type
Promise<Set<string>>
Usage
import { queryColumnSet } from "docs/database.ts";