What Is External Data?
External data is data that is stored outside the
current database. External data may be data that you store in another
Microsoft Access database, or it might be data that you store in a
multitude of other file formats—including ISAM (Indexed Sequential
Access Method), spreadsheet, ASCII, and more.
Access is an excellent front-end
product, which means that it provides a powerful and effective means of
presenting data—even data from external sources. You might opt to store
data in places other than Access for many reasons. You can most
effectively manage large databases, for example, on a back-end database
server such as Microsoft SQL Server. You might store data in a FoxPro,
dBASE, or Paradox file format because a legacy application written in
one of those environments is using the data. You might download text
data from a mainframe or midrange computer. Regardless of the reason
the data is stored in another format, it is necessary that you
understand how to manipulate this external data in Access applications.
With the capability to access data from other sources, you can create
queries, forms, and reports.
When you’re accessing external data, you
have two choices: You can import the data into an Access database or
you can access the data by linking to it from an Access database.
Importing the data is the optimum route (except with ODBC data
sources), but it is not always possible. If you can’t import external
data, you should link to external files because Microsoft Access
maintains a lot of information about these linked files. This optimizes
performance when manipulating the external files.
Importing, Linking, and Opening Files: When and Why
When
you import data into an Access table, Access makes a copy of the data
and places it in the Access table. After Access imports the data, it
treats the data like the data in any other native Access table. In
fact, neither you nor Access has any way of knowing from where the data
came. As a result, imported data offers the same performance and
flexibility as any other Access table data.
Linking to external data is quite different from
importing data. Linked data remains in its native format. By
establishing a link to the external data, you can build queries, forms,
and reports that present the data. After you create a link to external
data, the link remains permanently established unless you explicitly
remove it. The linked table appears in the Navigation Pane just like
any other Access table, except that its icon is different. In fact, if
the data source permits multiuser access, the users of an application
can modify the data as can the users of the applications written in the
data source’s native database format (such as FoxPro, dBASE, or
Paradox). The main difference between a linked table and a native table
is that you cannot modify a linked table’s structure from within Access.
Determining Whether to Import or Link
It is important that you understand when to import
external data and when to link to external data. You should import
external data in either of these circumstances:
If you are migrating an existing system into Access.
If
you want to use external data to run a large volume of queries and
reports, and you will not update the data. In this case, you want the
added performance that native Access data provides.
When you are migrating <$Imigration;importing
tables>an existing system to Access and you are ready to permanently
migrate test or production data into an application, you import the
tables into Access. You might also want to import external data if you
convert the data into ASCII format on a regular basis and you want to
use the data for reports. Instead of attempting to link to the data and
suffering the performance hits associated with such a link, you can
import the data each time you download it from the mainframe or
midrange computer.
You should link to external data in any of the following circumstances:
The data is used by a legacy application that requires the native file format.
The data resides on an ODBC-compliant database server.
You will access the data on a regular basis, making it prohibitive to keep the data up-to-date if you do not link to it.
Often, you won’t have the time or resources to
rewrite an application written in FoxPro, Paradox, or some other
language. You might be developing additional applications that will
share data with the legacy application, or you might want to use the
strong querying and reporting capabilities of Access rather than
develop queries and reports in the native environment.
If you link to the external data, users of existing
applications can continue to work with the applications and their data.
Access applications can retrieve and modify data without concern for
corrupting, or in any other way harming, the data.
If the data resides in an ODBC database such as
Microsoft SQL Server, you want to reap the data-retrieval benefits
provided by a database server. By linking to the ODBC data source, you
can take advantage of Access’s ease of use as a front-end tool and also
take advantage of client/server technology.
Finally, if you intend to access data on a regular
basis, linking to the external table provides you with ease of use and
performance benefits. After you create a link, in most cases, Access
treats the table just like any other Access table.
Looking at Supported File Formats
Microsoft Access enables you to import and link to files in these formats:
Microsoft Access databases (including versions earlier than Access 2010)
ODBC databases
HTML (Hypertext Markup Language) documents with <table> tags
XML (eXtensible Markup Language) documents (import and open only)
Microsoft Exchange and Outlook
dBASE III, dBASE IV, and dBASE 5.0
Paradox 3.x, 4.x, and 5.x
Microsoft Excel spreadsheets, versions 3.0, 4.0, 5.0, and 8.0
Lotus WKS, WK1, WK3, and WK4 spreadsheets (import and open only)
ASCII text files stored in a tabular format
