Calculating Mode within existing query to count rows in PHP and MySQL

I have a table rel with fields id refid cellid cat and some working PHP code to count the number of rows for each cellid and return this as an array for later use.

<?php
// connect to database
require($DOCUMENT_ROOT . "connect.php");
// count the number of relationships for each cell id
  $cell_array = array();
  if ($result = mysqli_query($link, "SELECT cellid, COUNT(*) totalcount, cat FROM rel GROUP BY cellid")) {
    if (mysqli_num_rows($result)) {
      while ($row = mysqli_fetch_assoc($result)) {
        $cell_array[$row["cellid"]] = $row['totalcount'];
      }
    }
  }
?>

I now want to add an additional function, calculate the Mode (most common value) of cat for each cellid and include that value in the array. If there is more than one mode or no mode then return 9 (cat can only have values of 1 to 8).

I've been reading around how to find the Mode and the common solution looks like this:

SELECT COUNT(*) frequency, cat, cellid
FROM rel
GROUP BY cat
ORDER BY COUNT(*) DESC
LIMIT 1

But this does not return the most common cat for each cellid just the most common cat in general.

I have been working using a sub-query

SELECT cellid, cat, COUNT( * ) 
FROM  `rel` 
GROUP BY cellid, cat

Which produces a row for each cellid cat combination, but I'm unsure how to then use this to find the total number of entries for each cellid and the the most common cat for each cellid

EDIT

I have made some further progress, I now have a working query to find the mode cat for each cellid

SELECT cellid, cat
FROM rel t
GROUP BY cellid, cat
HAVING cat = ( 
SELECT cat
FROM rel
WHERE cellid = t.cellid
GROUP BY cat
ORDER BY COUNT( * ) DESC , cat
LIMIT 1 )

But I am still unsure how to combine the two queries are the output from one cannot be used in the other.

Solution

You can combine them by joining them together:

select cell.*, mode.cat as modecat
from (SELECT cellid, COUNT(*) totalcount
      FROM rel
      GROUP BY cellid
     ) cell join
     (SELECT cellid, cat
      FROM rel t
      GROUP BY cellid, cat
      HAVING cat = (SELECT cat
                    FROM rel
                    WHERE cellid = t.cellid
                    GROUP BY cat
                    ORDER BY COUNT( * ) DESC , cat
                    LIMIT 1
                  )
     ) mode
     on cell.cellid = mode.cnt

This doesn't handle the case of multiple values. If you only wanted an arbitrary value, I would go with:

select cell.*,
       (select cat
        from rel
        where cell.cellid = rel.cellid
        group by cellid, cat
        order by COUNT(*) desc
        limit 1
       ) as mode
from (SELECT cellid, COUNT(*) totalcount
      FROM rel
      GROUP BY cellid
     ) cell

Identifying muliples is a bit trickier. It requires an additional aggregation in the subquery:

select cell.*,
       (select (case when min(cat) = max(cat) then min(cat) else 'MULTIPLE' end) as mode
        from (select cat, COUNT(*) as cnt
              from rel
              where cell.cellid = rel.cellid
              group by cellid, cat
             ) t
        group by cnt
        order by cnt desc
        limit 1
       ) mode
from (SELECT cellid, COUNT(*) totalcount
      FROM rel
      GROUP BY cellid
     ) cell