I need to create a stored procedure with a parameterised input. An input parameter is declared as a variable,this will hold the units that a user wants to insert from TableA into TableB. Using a CASE clause, within a stored procedure, it will first need to compare the input parameter against the avaiable units in TableA, let's say
in this case, an input of 3 units of Item_No (1235), is required to be inserted into TableB's 4 columns (refer below for field names). The stored procedure should be able to assign the units required base on the earliest Item_ID (1st Primary key comes first, on a first-in-first-out basis) from TableA, by first filtering a
number of sequential criteria. First it would search for the Item_No (1235) in TableA, next its available units,if both condition are met, next, insert a row of record into TableB's 4 columns, If units required is insufficient,it would loop for the next available units, then inserting the next row of records into TableB. An update procedure is also required to be created to update the units being used for the assignment in TableA (refer table below
for updated records for Item_No (1235).

The following are the fieldnames in TableA running on SQL Server 2000:

Dates as datetime
Item_ID as int (Primary key)
Item_No as int
Qty as int
Unit_Price as int

TableA consist of the following info :
================================================== ==
 Dates | Item_ID | Item_No | Qty | Unit_Price
================================================== ==
30/10/2007 | IT1000 | 1234 | 2 | 4
================================================== ==
30/10/2007 | IT1001 | 1235 | 2 | 6
================================================== ==
28/09/2007 | IT1002 | 1236 | 4 | 8
================================================== ==
01/11/2007 | IT1003 | 1235 | 2 | 2

INSERT Result in TableB after executing the stored procedure :
================================================== ===
 Dates | Item_ID | Item_No | Qty | Unit_Price
================================================== ===
30/10/2007 | IT1001 | 1235 | 2 | 6
================================================== ===
01/11/2007 | IT1003 | 1235 | 1 | 2

UPDATE Result in TableA after executing the stored procedure :
================================================== ====
 Dates | Item_ID | Item_No | Qty | Unit_Price
================================================== ====
30/10/2007 | IT1000 | 1234 | 2 | 4
================================================== ====
30/10/2007 | IT1001 | 1235 | 0 | 6
================================================== ====
28/09/2007 | IT1002 | 1236 | 4 | 8
================================================== ====
01/11/2007 | IT1003 | 1235 | 1 | 2

The following stored procedure was created:

Create procedure dbo.up_parmins_item
@Date_DT VARCHAR(22),
@Item_No INT,
@Qty_IT INT,
@Unit_Price_IT INT as

Declare @Item_ID INT

Select @Item_ID = Item_ID
From TableA
Where Item_No = @Item_No and Qty <> 0
-- If item not exist, then insert into TableA
If @Item_ID IS NULL
BEGIN
Insert into TableA
 ( Dates,Item_No,Qty,Unit_Price)
  Values(Getdate(),@Item_No,@Qty_IT,@Unit_Price_IT)
Set @Item_ID = @@IDENTITY
END
-- If item exist, and Qty is not 0, then insert into TableB and Update TableA
If @Item_ID IS NOT NULL and Qty <> 0
BEGIN
Set @Item_ID = Min(Item_ID),
Insert into TableB
 ( Dates ,Item_ID,Qty,Unit_Price)
 Values(Getdate(),@Item_ID,@Qty_IT,@Unit_Price_IT)
-- update quantity balance for Item_No (1235) in TableA
Update TableA
 Set Qty = (@Qty_IT - Qty)
 Where Item_No = @Item_No
END

If @@Error > 0
 BEGIN
 RAISERROR ('Update and Insert Items failed',16,1)
 RETURN 99
END

RETURN 0

My issue here is, how do I create a CASE clause or if-else statement to verify the quantity (Qty) available in TableA is sufficient for the input parameter (@Qty_IT) to be inserted into TableB? The procedure should be able to perform a loop in the CASE or If-else statement to allocate the quantity requested base on the earliest
Item_ID existing in TableA, and the next available units in the subsequent Item_ID (e.g. (1st) IT1001 - 2 units,(2nd) IT1003 - balance of 1 unit) If the units requested (input parameter) for an item, does not exist in TableA,a new record is then inserted into TableA for this new item. The update statement should be able to update the quantity for Item_No (1235) to 0 unit under Item_ID IT1001, and 1 unit under Item_ID IT1003.

Any suggestion to the above stored procedure would be helpful, Thanks guys!