tell me the sql - sql

Question

I have a table like this.
Table name : message
mid | mfrom | mto | msg
------------------------
1 | a | b | hi
2 | b | a | hello
3 | a | c | how are you
4 | c | a | fine
i am able to show all rows by
`$sql = mysql_query("SELECT *
FROM message
WHERE mto = '$to'
OR mfrom = '$to';");
while($row = mysql_fetch_array($sql))
{
echo $row['msg'];
}`
but i want to show only one result if mfrom and mto or mto and mfrom is equal. Such as if mfrom =a and mto=b or mfrom=b and mto=a. I want to show only one result from them like this
mid | mfrom | mto | msg
-------------------------
2 | b | a | hello
4 | c | a | fine
please tell me the query.

Answer 1

Your question is not clear because your "query" seems wrong ("mfrom and mto or mto and mfrom is equal" doesn't seem correct).
Nevertheless, as you named the table "message", I'm guessing that you want each message once even though it appears twice in the table.
What you could do is query from the table "twice" and then use the bigger of the ids:
SELECT first.* FROM message AS first LEFT JOIN message AS second
ON first.mfrom = second.mto AND first.mto = second.mfrom
WHERE first.mid > second.mid OR second.mid is NULL;
This would give you the result you requested. It would also give you any message that doesn't have a "partner" row.
If you want only matching messages, you can remove the "OR second.mid is NULL".