根据单个不同的列选择唯一的行

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

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

Quick one in TSQL

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;

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

SELECT * FROM emails GROUP BY email;