Question

I have a PL/SQL table with two columns: log_date (DATE) and value (FLOAT). The data is very fine-grained, the difference between log_dates could be a few milliseconds. The value changes over time. I want to find, using SQL, the maximum and minimum amount of time between log_dates it takes for value to increase.

Edit: Example

log_date | value
-------------------  
  15:00  |  10
  15:01  |  10
  15:02  |  11
  15:03  |  11
  15:04  |  11
  15:05  |  11
  15:06  |  12

Between 15:00 and 15:02 value increased BUT it also increased between 15:03 and 15:06 which took longer, and so I want a query that would return (in this case) ‘3 minutes’ (as a DATE or a NUMBER) – the longest amount of time it took for value to increase.

Answer 1

I can give you an answer in T-SQL, but I’m not sure what dialect you’re using. TBH, a loop here is the first thing that springs to mind (someone else may have a better way of doing it!):

DECLARE @temp TABLE ( log_date DATETIME, value FLOAT )
INSERT INTO @temp ( log_date, value ) SELECT log_date, value FROM <MyTableName>

DECLARE @diff TABLE ( time_diff INT, old_value FLOAT, new_value FLOAT )

-- the loop

DECLARE @prev_value FLOAT, 
        @cur_value FLOAT,
        @prev_log_date DATETIME,
        @cur_log_date DATETIME

WHILE EXISTS ( SELECT NULL FROM @temp )
BEGIN

    SELECT TOP 1 @cur_log_date = log_date, @cur_value = value
    FROM @temp
    ORDER BY log_date

    IF ( @prev_value IS NOT NULL AND @prev_log_date IS NOT NULL )
    BEGIN

        INSERT INTO @diff ( time_diff, old_value, new_value )
        SELECT DATEDIFF('ms', @prev_log_date, @cur_log_date ),
               @prev_value, @cur_value

    END

    SELECT @prev_log_date = @cur_log_date, @prev_value = @cur_value
    DELETE FROM @temp WHERE log_date = @cur_log_date

END

SELECT MAX(time_diff), MIN(time_diff) FROM @diffs

This way, you end up with a table of all differences that you can then query.

HTH