Alison Balteramp;#039;s Mastering Microsoft Office Access 1002003 [Electronic resources] نسخه متنی

Looking at Special Considerations

When dealing with different types of external files, various problems and issues arise. If you understand these stumbling blocks before they affect you, you will get a great head start in dealing with them.

dBASE

The major concerns you will have when dealing with dBASE files are deleted records, indexes, data types, and memo fields. When you delete a record from a dBASE table, Access does not remove it from the table. Instead, Access marks it for deletion. You must pack the database (a process in a dBASE table that removes deleted rows) for the records to actually be removed from the table. If records are deleted from a dBASE table using an Access application, the records are not removed. Because you cannot pack a dBASE database from within an Access application, the records still remain in the table. In fact, they are not automatically filtered from the Access table. To filter deleted records so that they cannot be seen within the Access application, the Deleted value in the \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Jet\4.0\Engines\Xbase section of the registry must be set to 01 (True).

Access can use the dBASE indexes to improve performance (Jet 4.0 Service Pack 5 the Borland Database Engine must be installed). After you link to a dBASE table and select an index, an INF file is created. This file has the same name as your dBASE database with an INF extension. It contains information about all the indexes being used. Here's an example of an INF:

[dBASE III]
NDX1=CUSTID.NDX
UNDX1=CUSTID.NDX

dBASE III is the database type identifier. NDX1 is an index number for the first index. The UNDX1 entry specifies a unique index.

The data types available in dBASE files are different from those available in Access files. It is important to understand how the field types are mapped. Table 19.4 shows how each dBASE data type is mapped to a Jet data type.

Table 19.4. Mapping of dBASE Data Types

Finally, make sure that you store the dBASE memo files in the same directory as the table. Otherwise, Access is unable to read the data in the memo file.

Text Data

When linking to an ASCII text file, Jet can determine the format of the file directly, or it can use a schema information file, which resides in the same directory as the text file. It always is named SCHEMA.INI, and it contains information about the format of the file, the column names, and the data types. The schema information file is optional for delimited files, but it is required for fixed length files. It is important to understand that ASCII files can never be opened for shared use.