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.
MD5 of a normalized email, `MD5(NORMALIZE_EMAIL(email))`: lowercase hex, 32 characters. Hashes of un-normalized emails won't join. The MD5 of an empty string, `d41d8cd98f00b204e9800998ecf8427e`, isn't a hash of any email and reads as NULL.
SHA-1 of a normalized email, `SHA1(NORMALIZE_EMAIL(email))`: lowercase hex, 40 characters. Hashes of un-normalized emails won't join. The SHA-1 of an empty string, `da39a3ee5e6b4b0d3255bfef95601890afd80709`, isn't a hash of any email and reads as NULL.