Question

I have a database table with one column being dates. However, some of the rows should share the same date but due to lag on insertion there’s a one second difference between them. The insert part has been fixed already but the current data in the table needs to be fixed as well.

As an example the following data is present:

2008-10-08 12:23:01   1   1   x
2008-10-08 12:23:01   1   2   y
2008-10-08 12:23:02   1   3   z

Now I want to update the last row in this example and set the date to ‘2008-10-08 12:23:01’.

Answer 1

The best way I can think of is writing an external script to do that. It’s tricky to determine which columns are correct and which should be updated without having more control over the grouping. Pseudo-code:

all_rows = SELECT * FROM table ORDER BY date
last_date = NULL
rows_to_update = []
for row in all_rows:
    if last_date is NULL or row.date - last_date > X seconds:
        set date to last_date for all rows from rows_to_update
        last_date = row.date
        rows_to_update = []
    else if row.date != last_date:
        rows_to_update += row

Alternatively, something like this could work, but you might need more than one run if want to handle cases where all three dates are different and you want to normalize two of them to the first one.

UPDATE
   tbl t,
   (SELECT
        t.date,
        (SELECT min(date)
         FROM tbl
         WHERE timestampdiff(SECOND,date,t.date) BETWEEN 1 AND 3) AS new_date
    FROM tbl t) t2
SET t.date=t2.new_date
WHERE t.date=t2.date AND t2.new_date IS NOT NULL