合并两个mysql COUNT（*）结果，不同的列

Question

I'm trying to combine the counts for the following queries:

  SELECT recipient AS otheruser, COUNT(*) 
    FROM privatemessages 
   WHERE sender = '$userid' 
GROUP BY recipient

  SELECT sender AS otheruser, COUNT(*) 
    FROM privatemessages 
   WHERE recipient = '$userid' 
GROUP BY sender

If there anyway to combine these two queries, and add the count(*) together for each 'otheruser'

Answer 1

The most straight forward way is to use SUM / Condition

SELECT
     CASE WHEN sender = '$userid' THEN recipient else sender END AS OtherUser,
     SUM(sender = '$userid') AS RCount,
     SUM(recipient = '$userid') AS SCount,
     COUNT(*) total
FROM
     privatemessages 
WHERE
     '$userid' IN (sender , recipient )
GROUP BY
     OtherUser

DEMO

Update

I cleaned up the SUM and GROUP BY statements thanks to deltab and Michael Buen

Answer 2

The most straightforward way is just to use 2 queries.
There is absolutely no harm in requesting 2 different sets of data using 2 queries yet not a single reason to do it with one.

Answer 3

You can combine them with UNION like this:

SELECT otheruser, COUNT(*) FROM
(SELECT recipient AS otheruser
FROM privatemessages 
WHERE sender = '$userid' 
GROUP BY recipient
UNION
SELECT sender AS otheruser
FROM privatemessages 
WHERE recipient = '$userid' 
GROUP BY sender)

Answer 4

SELECT recipient AS r, sender AS s, COUNT(total) // can't use same alias
FROM privatemessages
WHERE sender = '$userid' AND recipient = '$userid'
GROUP BY s;

Then in your results function, call the count

while ($row = $result) {
  echo $row['COUNT(total)'];
}

I believe this query should work as expected. The only issue with your original was the dual alias and your grouping of two different fields. This is possible in 2 seperate queries but combined, it can't be done (at least the alias, don't think so with GROUP BY).

Hope this works for you.

UPDATE

A great alternative to COUNT is to simply use num_rows ... Therefore, the query above would be:

SELECT recipient AS r, sender AS s // can't use same alias
FROM privatemessages
WHERE s = '$userid' AND r = '$userid'
GROUP BY s;

And then in your results function, call num_rows function:

mysql_num_rows($result);