Question

I have this table Test as Id, SomeValue, SomeText contains about a mill records, I altered the table to

ALTER TABLE Test
ADD ComputedColFloat FLOAT

all are set to null, fine! what I want is that for each row, the value of ComputedColFloat should be set to sum of values in column SomeValue upto that row. (The values in SomeValue are computed using RAND()!)

Say, if before adding the column, first five rows are

Id    SomeValue    SomeText
1     .91          text
2     .34          text
3     .52          text
4     .11          text
5     .68          text

what I want is to write a query that would update the table and set the added column to reflect the values as

Id    SomeValue    SomeText    ComputedColFloat
1     .91          text        .91   // .91
2     .34          text        1.25  // .91 + .34
3     .52          text        1.77  // .91 + .34 + .52
4     .11          text        1.88  // .91 + .34 + .52 + .11
5     .68          text        2.56  // .91 + .34 + .52 + .11 + .68 

I am using sql server 2008, and would prefer to do this from SSMS not from visual studio by C# or anything! (In C# in VS, I could loop, find the sum and then write a query that would update the records, but that’s not what I need, I need to do it from SSMS)
Also, if there are more than 1 way to do this, I would like to know what would be best performance wise, as I have about a mill records, I don’t wanna wait up forever! 🙁

Answer 1

You can create cursor for your table and update each row.

        DECLARE CUR_TEST CURSOR LOCAL For Select ID,SomeValue From test ORDER BY ID ASC
        DECLARE @TOTALSUM FLOAT
                    DECLARE @ID INT
        DECLARE @SOMEMVALUE FLOAT
        OPEN CUR_LOCAL
        SET @TOTALSUM=0
        SET @SOMEVALUE=0
        FETCH CUR_LOCAL INTO @ID,@SOMEVALUE
        WHILE @@FETCH_STATUS = 0 
        BEGIN
            SET @TOTALSUM=@TOTALSUM + @SOMEVALUE
            Update TEST SET ComputedColFloat =@TOTALSUM Where ID=@ID
            FETCH CUR_LOCAL INTO @ID,@SOMEVALUE
        END
        CLOSE CUR_LOCAL
        DEALLOCATE CUR_LOCAL

Before executing take a copy of your table to verify