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         


        

Answer 1

I'm not sure if following counts considering your constraints but it gets the job done.

Test data

DECLARE @Customers TABLE (Date DATETIME, Customer INTEGER, Value INTEGER)

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

Query

SELECT  c.Date
        , c.Customer
        , Value = COALESCE(c.Value, cprevious.Value, 0)
FROM    @Customers c
        INNER JOIN (
          SELECT  c.Date
                  , c.Customer
                  , MaxDate = MAX(cdates.Date)
          FROM    @Customers c
                  LEFT OUTER JOIN (
                    SELECT  Date
                            , Customer
                    FROM    @Customers
                  ) cdates ON cdates.Date < c.Date AND cdates.Customer = c.Customer
          GROUP BY
                  c.Date, c.Customer
        ) cmax ON cmax.Date = c.Date AND cmax.Customer = c.Customer                  
        LEFT OUTER JOIN @Customers cprevious ON cprevious.Date = cmax.MaxDate AND cprevious.Customer = cmax.Customer
ORDER BY
        1, 2, 3        

Update statement

UPDATE  @Customers 
SET     Value = c2.Value 
OUTPUT  Inserted.* 
FROM    @Customers c 
        INNER JOIN ( 
          SELECT  c.Date
                  , c.Customer
                  , Value = COALESCE(c.Value, cprevious.Value, 0)
          FROM    @Customers c
                  INNER JOIN (
                    SELECT  c.Date
                            , c.Customer
                            , MaxDate = MAX(cdates.Date)
                    FROM    @Customers c
                            LEFT OUTER JOIN (
                              SELECT  Date
                                      , Customer
                              FROM    @Customers
                            ) cdates ON cdates.Date < c.Date AND cdates.Customer = c.Customer
                    GROUP BY
                            c.Date, c.Customer
                  ) cmax ON cmax.Date = c.Date AND cmax.Customer = c.Customer                  
                  LEFT OUTER JOIN @Customers cprevious ON cprevious.Date = cmax.MaxDate AND cprevious.Customer = cmax.Customer
        ) c2 ON c2.Date = c.Date 
                AND c2.Customer = c.Customer