Question

For example, if Column A and Column B have values:

+---+---+
| A | B |
+---+---+
| 2 | 1 |
| 5 | 1 | 
| 6 | 1 |
| 1 | 2 |
| 5 | 2 |
| 0 | 2 |
| 2 | 3 |
| 7 | 3 |
| 4 | 3 |
| 5 | 4 |
+---+---+

From each group of B, I want to get the highest number from A. However, I don’t want to include results where the number in B is higher, yet has a smaller A value than the previous one. I know this doesn’t make sense in words, but this is what I want the final result to look like:

+---+---+
| A | B |
+---+---+
| 6 | 1 |
| 7 | 3 |
+---+---+

So far I have something like “select max(a), b from table1 group by b” but this doesn’t omit the ones where B is higher but the max A is smaller. I know that I could just peruse the results of that query in PHP and remove the ones where the A value is smaller than the previous A value, but I want to put it all in the mysql query if possible.

Answer 1

A simpler solution would be to use a variable to store the value of a from the previous row and make the comparison on each iteration. This also accounts for the case where you might have gaps in the b column, where numbers aren’t exactly in perfect sequential order:

SELECT @val:=a.a AS a, a.b
FROM
(
  SELECT MAX(a) AS a, b
  FROM tbl
  GROUP BY b
) a
WHERE a.a > IFNULL(@val,-1)