Question

This is my table structure:

CREATE table Credit(id integer, organisationid int, availableCredit int)

INSERT INTO Credit VALUES (1, 1, 1000)
INSERT INTO Credit VALUES (2, 1, 100)
INSERT INTO Credit VALUES (3, 2, 600)
INSERT INTO Credit VALUES (4, 2, 400)

I have to reduce the available credit column value, I have amount 1050 with me. I need to reduce 1050 from credit table where organisation id = 1. Here the organisation Id 1 have 1100 available credit in total. The condition is the first inserted credit row should be updated first and then the rest (FIFO model), also the updation should happen only for organisationId = 1.

How do we update this using a single or multiple update statement?

Any suggestions?

Answer 1

Unfortunately, this is a rather messy thing to do in T-SQL – you’ll need something like a loop (cursor or WHILE statement).

With this code here, I can get it to run – it’s not pretty, but it works.

-- you want to reduce the total credits by this amount   
DECLARE @AmountToReduce INT = 1050

-- temporary variables    
DECLARE @ID INT, @AvailableCredit INT

-- get the first row from dbo.Credit that still has availableCredit - ordered by id
SELECT TOP 1 @ID = id, @AvailableCredit = availableCredit
FROM dbo.Credit
WHERE availableCredit > 0 AND organisationId = 1
ORDER BY id 

-- as long as we still have credit to reduce - loop..   
WHILE @AmountToReduce > 0 AND @ID IS NOT NULL
BEGIN
    -- if we need to remove the complete availableCredit - do this UPDATE
    IF @AmountToReduce > @AvailableCredit
    BEGIN
        UPDATE dbo.Credit
        SET availableCredit = 0
        WHERE id = @ID

        SET @AmountToReduce = @AmountToReduce - @AvailableCredit
    END
    ELSE BEGIN
            -- if the amount to reduce left is less than the availableCredit - do this UPDATE
        UPDATE dbo.Credit
        SET availableCredit = availableCredit - @AmountToReduce
        WHERE id = @ID

        SET @AmountToReduce = 0
    END

    -- set @ID to NULL to be able to detect that there's no more rows left
    SET @ID = NULL

    -- select the next "first" row with availableCredit > 0 to process    
    SELECT TOP 1 @ID = id, @AvailableCredit = availableCredit
    FROM dbo.Credit
    WHERE availableCredit > 0 AND organisationId = 1
    ORDER BY id 
END