在MySQL单个表中找到重复的值

SELECT
    id,
    firstName,
    lastName,
    email,
    COUNT(*)
FROM
    contacts
GROUP BY
    firstName
HAVING
    COUNT(firstName) > 1;

SELECT
    id,
    firstName,
    lastName,
    email,
    COUNT(*)
FROM
    contacts
GROUP BY
    firstName,
    lastName,
    email
HAVING
    COUNT(firstName) > 1
AND COUNT(lastName) > 1
AND COUNT(email) > 1;