找重复值

方便演示，我们创建一个名为contacts表，其中包含四个列：id，first_name，last_name和email。

表中数据如下

| id | first_name | last_name       | email                          |
+----+------------+-----------------+--------------------------------+
|  1 | Carine     | Schmitt         | carine.schmitt@qq.com          |
|  2 | Jean       | King            | jean.king@yiibai.com               |
|  3 | Peter      | Ferguson        | peter.ferguson@google.com      |
|  4 | Janine     | Labrune         | janine.labrune@aol.com         |
|  5 | Jonas      | Bergulfsen      | jonas.bergulfsen@mac.com       |
|  6 | Janine     | Labrune         | janine.labrune@aol.com         |
|  7 | Susan      | Nelson          | susan.nelson@qq.com            |
|  8 | Zbyszek    | Piestrzeniewicz | zbyszek.piestrzeniewicz@qq.com |
|  9 | Roland     | Keitel          | roland.keitel@yahoo.com        |
| 10 | Julie      | Murphy          | julie.murphy@yahoo.com         |
| 11 | Kwai       | Lee             | kwai.lee@google.com            |
| 12 | Jean       | King            | jean.king@qq.com               |
| 13 | Susan      | Nelson          | susan.nelson@qq.comt           |
| 14 | Roland     | Keitel          | roland.keitel@yahoo.com 

在一列中找到重复的值

SELECT 
    email, 
    COUNT(email)
FROM
    contacts
GROUP BY email
HAVING COUNT(email) > 1;

输出如下

+-------------------------+--------------+
| email                   | COUNT(email) |
+-------------------------+--------------+
| janine.labrune@aol.com  |            2 |
| roland.keitel@yahoo.com |            2 |
+-------------------------+--------------+
2 rows in set

在多个列中查找重复值

SELECT 
    first_name, COUNT(first_name),
    last_name,  COUNT(last_name),
    email,      COUNT(email)
FROM
    contacts
GROUP BY 
    first_name , 
    last_name , 
    email
HAVING  COUNT(first_name) > 1
    AND COUNT(last_name) > 1
    AND COUNT(email) > 1;

输出

+------------+-------------------+-----------+------------------+-------------------------+--------------+
| first_name | COUNT(first_name) | last_name | COUNT(last_name) | email                   | COUNT(email) |
+------------+-------------------+-----------+------------------+-------------------------+--------------+
| Janine     |                 2 | Labrune   |                2 | janine.labrune@aol.com  |            2 |
| Roland     |                 2 | Keitel    |                2 | roland.keitel@yahoo.com |            2 |
+------------+-------------------+-----------+------------------+-------------------------+--------------+
2 rows in set