mysql - 如何增加 MySQL 中列值的出现次数

Question

我有以下列名称:

customer_email

increment_id

other_id(假名)

created_at

increment_id和 other_id将是独一无二的，customer_email会有重复。返回结果时，我想知道电子​​邮件出现的次数。

对于每一行，我想知道 customer_email 的次数是多少到目前为止，值(value)已经显现。会有order by条款结尾为created_at字段，我还计划添加一个 where 子句 where occurrences < 2
我正在查询 > 500 万行，但性能并不是很重要，因为我将把它作为生产中只读副本数据库的报告运行。在我的用例中，我会为了健壮性而牺牲性能。

| customer_email | incremenet_id | other_id | created_at          | occurances <- I want this |
|----------------|---------------|----------|---------------------|---------------------------|
| joe@test.com   | 1             | 81       | 2019-11-00 00:00:00 | 1                         |
| sue@test.com   | 2             | 82       | 2019-11-00 00:01:00 | 1                         |
| bill@test.com  | 3             | 83       | 2019-11-00 00:02:00 | 1                         |
| joe@test.com   | 4             | 84       | 2019-11-00 00:03:00 | 2                         |
| mike@test.com  | 5             | 85       | 2019-11-00 00:04:00 | 1                         |
| sue@test.com   | 6             | 86       | 2019-11-00 00:05:00 | 2                         |
| joe@test.com   | 7             | 87       | 2019-11-00 00:06:00 | 3                         |

Answer 1

您可以在早期版本的 MySQL 中使用变量:

select t.*,
       (@rn := if(@ce = customer_email, @rn + 1,
                  if(@ce := customer_email, 1, 1)
                 )
       ) as occurrences
from (select t.*
      from t
      order by customer_email, created_at
     ) t cross join
     (select @ce := '', @rn := 0) params;

在 MySQL 8+ 中，我会推荐 row_number() :

select t.*,
       row_number() over (partition by customer_email order by created_at) as occurrences
from t;