MySQL: Find max consecutive rows in a table based on value - mysql

Question

Using MySQL, I am trying to find the highest number of consecutive rows in a table based on a value. For the sake of simplicity, my table looks like this:
+----+-------+
| ID | VALUE |
+----+-------+
| 1 | A |
| 2 | B |
| 3 | A |
| 4 | A |
| 5 | B |
| 6 | B |
| 7 | A |
| 8 | A |
| 9 | A |
| 10 | B |
+----+-------+
In this example, if I wanted the highest number of consecutive rows for 'A', I would get 3. For 'B', I would get 2. Even returning a result set of the counts of consecutive rows for 'A' would be preferable. I am newer to SQL so hints would be appreciated too. Any suggestions?

Answer 1

You can do it using variables:
SELECT VALUE, MAX(cnt) AS maxCount
FROM (
SELECT VALUE, COUNT(grp) AS cnt
FROM (
SELECT ID, VALUE, rn - rnByVal AS grp
FROM (
SELECT ID, VALUE,
#rn := #rn + 1 AS rn,
#rnByVal := IF (#val = VALUE,
IF (#val := VALUE, #rnByVal + 1, #rnByVal + 1),
IF (#val := VALUE, 1, 1)) AS rnByVal
FROM mytable
CROSS JOIN (SELECT #rn := 0, #rnByVal := 0, #val := '') AS vars
ORDER BY ID) AS t
) AS s
GROUP BY VALUE, grp ) AS u
GROUP BY VALUE
Variables #rn and #rnByVal are used in order to simulate ROW_NUMBER window function, currently not available in MySQL. The second variable (#rnByVal) performs a count over VALUE partitions.
Using #rn - #rnByVal in an outer query we can calculate grp field, which identifies islands of consecutive rows having the same VALUE. Performing a GROUP BY on VALUE, grp we can calculate the population of these islands and, finally, in the outermost query, get the max population per VALUE.
Demo here