ACC: Behavior of Query Differs for Byte Data in MS Access 95/97

• Microsoft Access versions 7.0, 97

SYMPTOMS

Novice: Requires knowledge of the user interface on single-user computers.

When you create an update query that calculates values for a numeric field, and you set the Field Size to Byte, you may not achieve the results you expect when the calculated value in the field is larger than what the Byte size data type can hold. The following behavior will occur if this happens.

In Microsoft Access 7.0 and 97

The query updates the value in the field to the calculated value minus 256.

In Microsoft Access 2.0

The query does not update the field, and you receive a conversion error message.

CAUSE

The Byte data type is used to store small, positive integers ranging from 0 to 255.

RESOLUTION

Use a different field size for your calculated field if you are going to be creating calculations that exceed 255.

MORE INFORMATION

Steps to Reproduce Behavior

1. Start Microsoft Access 7.0 or 97.

2. Create a new table and save it as Table1. Add the following field to the table:

         Table: Table1
         --------------------
         Field name: Field1
            Data Type: Number
            Field size: Byte
   
   

3. Enter the following data in the first three rows:

         10, 15, 20
   

4. Close the table.

5. Create an update query based on Table1. To do so, follow these steps:

       a. In the Database window, click the Query tab, and then click New.
   

       b. In the New Query box, click Design view, and then click OK.
   

       c. In the Show Table box, select Table1, click Add, and then click
          Close.
   

       d. Add Field1 to the query grid.
   

       e. On the Query menu, click Update Query.
   

       f. In the Update To row, type [Field1] * 2.
   

6. Run the query four times. Click Yes to each confirmation message.

   Note that when the value that should be returned is greater than the Byte data type size limitation of 255, an incorrect value is returned to the field. In the above example, the values that are returned each time the query runs are as follows:

         Query        Values that are returned
         -----        ------------------------
          One         20, 30, 40
          Two         40, 60, 80
          Three       80, 120, 160
          Four        160, 240, 64
   
   

REFERENCES