Pandas dropna to WHERE IS NOT NULL

Dropping blank strings is a second test. SQL NULL and '' are not the same.

SELECT * FROM Customers
WHERE email IS NOT NULL AND LTRIM(RTRIM(email)) <> '';

how="all" is WHERE NOT (a IS NULL AND b IS NULL).

Canonical translation

SELECT * FROM Customers
WHERE email IS NOT NULL AND LTRIM(RTRIM(email)) <> '';