[sql-server] select unique rows based on single distinct column

I want to select rows that have a distinct email, see the example table below:

+----+---------+-------------------+-------------+
| id | title   | email             | commentname |
+----+---------+-------------------+-------------+
|  3 | test    | [email protected]   | rob         |
|  4 | i agree | [email protected]   | rob         |
|  5 | its ok  | [email protected]   | rob         |
|  6 | hey     | [email protected]   | rob         |
|  7 | nice!   | [email protected] | simon       |
|  8 | yeah    | [email protected]  | john        |
+----+---------+-------------------+-------------+

The desired result would be:

+----+-------+-------------------+-------------+
| id | title | email             | commentname |
+----+-------+-------------------+-------------+
|  3 | test  | [email protected]   | rob         |
|  7 | nice! | [email protected] | simon       |
|  8 | yeah  | [email protected]  | john        |
+----+-------+-------------------+-------------+

Where I don't care which id column value is returned. What would be the required SQL?

I'm assuming you mean that you don't care which row is used to obtain the title, id, and commentname values (you have "rob" for all of the rows, but I don't know if that is actually something that would be enforced or not in your data model). If so, then you can use windowing functions to return the first row for a given email address:

select
    id,
    title,
    email,
    commentname

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

from YourTable
) source

where RowNbr = 1

If you are using MySql 5.7 or later, according to these links (MySql Official, SO QA), we can select one record per group by with out the need of any aggregate functions.

So the query can be simplified to this.

select * from comments_table group by commentname;

Try out the query in action here

Since you don't care which id to return I stick with MAX id for each email to simplify SQL query, give it a try

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