algorithm/query to get statistical data from table using php/mysql - php

Question

I have the following table structure:
CREATE TABLE test(
myID INT,
num1 INT,
num2 INT,
num3 INT,
PRIMARY KEY (myID)
)engine=innodb;
now i have the following data in the table:
myID num1 num2 num3
1 15 27 98
2 27 38 66
3 15 27 77
now i need to run 2 queries, first query runs to select all numbers and on PHP side i count the number of times each number appeared (frequencies), the second query should select the second most frequent number.
First query:
$numfreq = PDO->prepare('
SELECT num1, num2, num3
FROM test
');
$numfreq->execute();
$allNums = array();
while ($row = $numfreq->fetch(PDO::FETCH_ASSOC)) {
$allNums[] = intval($row['num1']);
$allNums[] = intval($row['num2']);
$allNums[] = intval($row['num3']);
}
$numFrequencies = array_count_values($allNums);
arsort($numFrequencies);
this correctly returns frequencies of each number from the table. Now for the second part
THIS IS WHERE I NEED HELP:
I get the most frequent appeared number in this case its 27 since its frequency is 3, I need to select that 1 number that appears the most next to 27 means i need to get somehow number 15 since it appears twice next to 27.
i can probably figure out algorithm on PHP side but i was wondering if its possible to do it using query?
so the final result would be:
most frequent number: 27
most frequent number 27 combined with 15 appears 2 times and is most frequent combination.

Answer 1

select val, count(val) as frequency
from
(select num1 as val from test
union all
select num2 as val from test
union all
select num3 as val from test
) as b
group by val
order by frequency desc
limit 2
Sqlfiddle here.
The inner query converts the three columns into a result set with just one column - highlight the inner query, and you'll see how it works. We then use that result set as the source for the counting/ordering query.