Combining select distinct and order by - mysql

Question

This is my SQL query :
SELECT DISTINCT dest
FROM messages
WHERE exp = ?
UNION
SELECT DISTINCT exp
FROM messages
WHERE dest = ?
With this query, I get all the messages I've sent or received. But in my table messages, i have a field timestamp, and i need, with this query, add an order by timestamp... but how ?

Answer 1

You can do this without a union:
SELECT (case when exp = ? then dest else exp end), timestamp
FROM messages
WHERE exp = ? or dest = ?;
Then to get the most recent message for each participant, use group by not distinct:
SELECT (case when exp = ? then dest else exp end) as other, max(timestamp)
FROM messages
WHERE exp = ? or dest = ?
group by (case when exp = ? then dest else exp end)
order by max(timestamp) desc;

Answer 2

SELECT *
FROM
( SELECT
col1 = dest,
col2 = MAX(timestampCol)
FROM
messages
WHERE
exp = ?
GROUP BY
dest
UNION
SELECT
col1 = exp,
col2 = MAX(timestampCol)
FROM
messages
WHERE
dest= ?
GROUP BY
exp
) tbl
ORDER BY col2
This should return only one row per distinct exp / dest though I'm sure this could probably be done without a union; The GROUP BY will only get the most recent one.
Updated SQL: Given that it is possible for an exp on one record to equal a dest on the same or another record.
SELECT
CASE WHEN exp = ? THEN dest ELSE exp END AS col1
,MAX(timestampCol) AS col2
FROM
messages
WHERE
exp = ?
OR dest = ?
GROUP BY
(CASE WHEN exp = ? THEN dest ELSE exp END)
ORDER BY
MAX(timestampCol) DESC;
You might want to consider adding an SQL Fiddle with some dummy data to allow users to better help you.

Answer 3

SELECT *
FROM
(
SELECT DISTINCT dest AS Column1, timestamp AS Column2
FROM messages
WHERE exp = ?
UNION
SELECT DISTINCT exp AS Column1, timestamp AS Column2
FROM messages
WHERE dest = ?
) tbl
ORDER BY Column2

Answer 4

This query will help you in getting records order by timestamp
SELECT tbl.mailId, Max(timestampColumn) MostRecentDate
FROM
(
SELECT exp AS mailId, Max(timestampCol) AS timestampColumn
FROM Employeeatd
WHERE dest = ?
GROUP BY exp
UNION
SELECT Dept AS abc, Max(timestampCol) AS timestampColumn
FROM Employeeatd
WHERE exp = ?
GROUP BY Dept
) tbl
GROUP BY mailId
ORDER BY Max(timestampColumn) desc