get number of occurrences between two columns same table

Question

I have a problem on how to fetch the number of occurrences of a value in between two columns in MySQL.

id1 col1 col2
2    5      3
3    3      4
4    2      1
5    1      3
6    null   2

How am I able to get the number of occurences between the two columns like the following?

value     occurrence
3                3
1                2
2                2
4                1
5                1

Answer 1

You could union all the columns and then apply a count aggregate function:

SELECT   val, COUNT(*) AS occurrence
FROM     (SELECT col1 AS val
          FROM   mytable
          UNION ALL
          SELECT col2 AS val
          FROM   mytable) x
GROUP BY val
ORDER BY occurrence DESC

Answer 2

Depending on the actual data (number of rows per value) pre-aggregating might be more efficient, simply try it out:

SELECT   val, SUM(occurrence) AS occurrence
FROM     (SELECT col1 AS val, COUNT(*) AS occurrence
          FROM   mytable
          UNION ALL
          SELECT col2 AS val, COUNT(*) AS occurrence
          FROM   mytable) x
GROUP BY val
ORDER BY occurrence DESC