MySQL datetime comparison with previous row

问题

I have a table with two Date columns. DATE1 is sometimes NULL and sometimes contains duplicate values. DATE2 is always populated and unique. My table is sorted by latest DATE2 date.

I'd like to create a new date column where DATE1 will be selected unless its value is duplicated from the next row or it's NULL. In this case, I want to take the value of DATE2. I also need two boolean columns that tell me when either of those conditions were met. Let me demonstrate using an example so it's clearer.

In the table below, row 5 and 6 have a value of Jul 27, 2011 so I'd like to set the new date column of row 5 to Aug 4, 2011 (which is DATE2). In row 3, the value of DATE1 is NULL so I want to take the value of DATE2.

I've tried a few inner select statements but can't get this to work. Any ideas?

My table as it currently stands in the database:

    Row     DATE1           DATE2
    --------------------------------------
    1       Oct 10, 2011    Nov 13, 2011    
    2       Oct 10, 2011    Oct 10, 2011    
    3       NULL            Oct 8, 2011     
    4       Aug 12, 2011    Aug 12, 2011    
    5       Jul 27, 2011    Aug 4, 2011     
    6       Jul 27, 2011    Jul 28, 2011    
    7       Jul 1, 2011     Jul 26, 2011    
    8       May 24, 2011    Jun 13, 2011

What I expect the final result to look like:

    Row     FINAL_DATE      DATE1_DUPLICATE   DATE1_WAS_NULL
    ----------------------------------------------------------
    1       Nov 13, 2011    TRUE              FALSE   
    2       Oct 10, 2011    FALSE             FALSE
    3       Oct 8, 2011     FALSE             TRUE
    4       Aug 12, 2011    FALSE             FALSE
    5       Aug 4, 2011     TRUE              FALSE 
    6       Jul 27, 2011    FALSE             FALSE
    7       Jul 1, 2011     FALSE             FALSE
    8       May 24, 2011    FALSE             FALSE

Thanks so much!

回答1:

This can be handled with sequential scanning of the table and using MySQL variables. You can test in (updated) SQL-fiddle:

SELECT date2
     , dd 
     , DATE_FORMAT(dd, '%b %e, %Y') AS final_date
     , date1_duplicate
     , date1_was_null
FROM
( SELECT date2
       , COALESCE( (date1 = @d OR date1 = @prev), FALSE)
           AS date1_duplicate
       , (date1 IS NULL)               AS date1_was_null
       , @d := CASE WHEN (date1 = @d OR date1 = @prev) 
                 THEN date2 
                 ELSE COALESCE(date1, date2) 
               END AS dd
       , @prev := date1 AS pre
  FROM tableX AS t
    CROSS JOIN
      ( SELECT @d := DATE('1000-01-01')
             , @prev := @d 
      ) AS dummy
  ORDER BY date2 ASC 
) AS tmp
ORDER BY date2 DESC
;

来源：https://stackoverflow.com/questions/10079190/mysql-datetime-comparison-with-previous-row

标签

mysql

window-functions