使用SQL获取以前的日期

Question

My query is

SELECT 
    matno,
    MAT_NAME,
    MAX(SWITCH(deldate=? ,ORDCASES )) AS [1/1/2014],
    MAX(SWITCH(deldate=DateAdd("d", -1, ?),ORDCASES)) AS [previous_day] 
FROM invorder 
WHERE (invorder.strno =54009) OR ([invorder.deldate] IS NULL) 
GROUP BY  matno,MAT_NAME;

Here I can just check one before date of selected date,but if their are no records with previous date the I should decrement date by one more day and check until I find the records. Can someone help me how can I decrement and find the previous date?

Answer 1

Without understanding the exact case, I would suggest an approach like:

SELECT MAX(date) 
FROM myTable
WHERE date < '2014-01-01' --> Replace this as needed

This would result in the previous date with a record . Without the need of checking each and every day by substraction.

I've tried to match your case, try this:

SELECT   invorder.matno,
         invorder.MAT_NAME,
         MAX(invorder.date) AS this_record_date,
         MAX(previous.date) AS prev_record_date
FROM     invorder 
JOIN     invorder AS previous 
                  ON invorder.matno = previous.matno 
                  AND invorder.strno = previous.strno 
                  AND invorder.date > previous.date
WHERE    invorder.strno =54009
GROUP BY invorder.matno, 
         invorder.MAT_NAME

This selects the maximum record by date of strno 54009, it's matno , MAT_NAME and date . This row is extended with the maximum date of the same matno and strno excluding the date we already selected (so pretty much the second most maximum date).