SHA-256 of a normalized email, `SHA2(NORMALIZE_EMAIL(email), 256)`: lowercase hex, 64 characters. Hashes of un-normalized emails won't join. The SHA-256 of an empty string, `e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855`, isn't a hash of any email and reads as NULL.