SQL: How to fill empty cells with previous row value?

Question

I need to produce the column "required" in the following table using SQL without using loops and correlated sub queries. Is this possible in SQL 2008?

Date    Customer   Value   Required   Rule
20100101       1      12         12
20100101       2                  0   If no value assign 0
20100101       3      32         32
20100101       4      42         42
20100101       5      15         15
20100102       1                 12   Take last known value
20100102       2                  0   Take last known value
20100102       3      39         39
20100102       4                 42   Take last known value
20100102       5      16         16
20100103       1      13         13
20100103       2      24         24
20100103       3                 39   Take last known value
20100103       4                 42   Take last known value
20100103       5      21         21
20100104       1      14         14
20100104       2                 24   Take last known value
20100104       3                 39   Take last known value
20100104       4      65         65
20100104       5      23         23

Basically I am filling empty "Value" cells with last know value for that customer. Remember the last row may not have a valid value, so you will have to pick it from the row before that with a valid value.

Answer 1

Faiz,

how about the following query, it does what you want as far as I understand it. The comments explain each step. Take a look at CTEs on Books Online. This example could even be changed to use the new MERGE command for SQL 2008.

/* Test Data & Table */
DECLARE @Customers TABLE
    (Dates datetime,
     Customer integer,
     Value integer) 

    INSERT  INTO @Customers
    VALUES  ('20100101', 1, 12),
        ('20100101', 2, NULL),
        ('20100101', 3, 32),
        ('20100101', 4, 42),
        ('20100101', 5, 15),
        ('20100102', 1, NULL),
        ('20100102', 2, NULL),
        ('20100102', 3, 39),
        ('20100102', 4, NULL),
        ('20100102', 5, 16),
        ('20100103', 1, 13),
        ('20100103', 2, 24),
        ('20100103', 3, NULL),
        ('20100103', 4, NULL),
        ('20100103', 5, 21),
        ('20100104', 1, 14),
        ('20100104', 2, NULL),
        ('20100104', 3, NULL),
        ('20100104', 4, 65),
        ('20100104', 5, 23) ;

/* CustCTE - This gives us a RowNum to allow us to build the recursive CTE CleanCust */
WITH    CustCTE
          AS (SELECT    Customer,
                        Value,
                        Dates,
                        ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY Dates) RowNum
              FROM      @Customers),

/* CleanCust - A recursive CTE. This runs down the list of values for each customer, checking the Value column, if it is null it gets the previous non NULL value.*/
        CleanCust
          AS (SELECT    Customer,
                        ISNULL(Value, 0) Value, /* Ensure we start with no NULL values for each customer */
                        Dates,
                        RowNum
              FROM      CustCte cur
              WHERE     RowNum = 1
              UNION ALL
              SELECT    Curr.Customer,
                        ISNULL(Curr.Value, prev.Value) Value,
                        Curr.Dates,
                        Curr.RowNum
              FROM      CustCte curr
              INNER JOIN CleanCust prev ON curr.Customer = prev.Customer
                                           AND curr.RowNum = prev.RowNum + 1)

/* Update the base table using the result set from the recursive CTE */
    UPDATE trg
    SET Value = src.Value
    FROM    @Customers trg
    INNER JOIN CleanCust src ON trg.Customer = src.Customer
                                AND trg.Dates = src.Dates

/* Display the results */
SELECT * FROM @Customers