基于单个不同列选择唯一行

Question

我想选择具有distinct email 的行，请参见下面的示例表：

+----+---------+-------------------+-------------+
| id | title   | email             | commentname |
+----+---------+-------------------+-------------+
|  3 | test    | rob@hotmail.com   | rob         |
|  4 | i agree | rob@hotmail.com   | rob         |
|  5 | its ok  | rob@hotmail.com   | rob         |
|  6 | hey     | rob@hotmail.com   | rob         |
|  7 | nice!   | simon@hotmail.com | simon       |
|  8 | yeah    | john@hotmail.com  | john        |
+----+---------+-------------------+-------------+

期望的结果是：

+----+-------+-------------------+-------------+
| id | title | email             | commentname |
+----+-------+-------------------+-------------+
|  3 | test  | rob@hotmail.com   | rob         |
|  7 | nice! | simon@hotmail.com | simon       |
|  8 | yeah  | john@hotmail.com  | john        |
+----+-------+-------------------+-------------+

我不在乎返回哪个 id 列值。 所需的 SQL 是什么？

Answer 1

如果您使用的是MySql 5.7 或更高版本，根据这些链接（MySql Official、SO QA），我们可以在每个group by 中选择一条记录，而无需任何聚合函数.

所以查询可以简化成这样。

select * from comments_table group by commentname;

在行动中试用查询here

Answer 2

TSQL 中的快速操作

SELECT a.*
FROM emails a
INNER JOIN 
  (SELECT email,
    MIN(id) as id
  FROM emails 
  GROUP BY email 
) AS b
  ON a.email = b.email 
  AND a.id = b.id;

Answer 3

由于您不在乎返回哪个 id，我坚持为每封电子邮件使用 MAX id 以简化 SQL 查询，请尝试一下

;WITH ue(id)
 AS
 (
   SELECT MAX(id)
   FROM table
   GROUP BY email
 )
 SELECT * FROM table t
 INNER JOIN ue ON ue.id = t.id

Answer 4

我假设您的意思是您不关心用于获取 title、id 和 commentname 值的行（您对所有行都有“rob”，但我不关心'不知道这是否真的会在您的数据模型中强制执行）。如果是这样，那么您可以使用窗口函数返回给定电子邮件地址的第一行：

select
    id,
    title,
    email,
    commentname

from
(
select 
    *, 
    row_number() over (partition by email order by id) as RowNbr 

from YourTable
) source

where RowNbr = 1