ACC2000: How to Use Code to Change a Field's Data Type at Run Time (210304)

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To create and use the sChangeDataType procedure, follow these steps:

CAUTION: If you follow the steps in this example, you modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file and follow these steps on a copy of the database.

1. Start Microsoft Access, and then open the sample database Northwind.mdb or the sample project NorthwindCS.adp.
2. Note the data type of the Extension field in the Employees table.
3. Create a new module, and then type Option Explicit in the Declarations section if it is not already there.
4. Type or paste the following code in the module:

   Sub sChangeDataType()
       Dim cnn As ADODB.Connection
       Dim strSQL As String
       Set cnn = CurrentProject.Connection
       strSQL = "ALTER TABLE [Employees] ALTER COLUMN Extension int"
       cnn.Execute strSQL
   End Sub
   					

5. On the View menu, click Immediate Window.
6. In the Immediate window, type the following line, and then press ENTER:

   sChangeDataType

   Note the changes to the Extension field in the table's design.

Additional Notes About the Code

• If the code cannot convert the values in the field to the new data type, you receive the following message, and the data type change will not occur:
  
  In an Access database (.mdb):
  Data type mismatch in criteria expression.
  In an Access project (.adp):
  Syntax error converting the [olddatatype] value [value] to a column of data type [newdatatype].
• If you set a column size smaller than the data currently in the field, you receive the following message, and the data type change will not occur:
  
  In an Access database (.mdb):
  The field is too small to accept the amount of data you attempted to add. Try inserting or pasting less data.
  In an Access project (.adp):
  String or binary data would be truncated.
• Microsoft Access prohibits the conversion of any field data type to the AutoNumber data type. You receive the following message if you try to change the column by using the IDENTITY keyword:
  
  In an Access database (.mdb):
  Syntax error in ALTER TABLE statement.
  In an Access project (.adp):
  Incorrect syntax near the keyword 'IDENTITY'.
• Data type changes can take a long time if you have a lot of data in the table.