Working with Data Types .................................................................................................
583
Caution
Access is capable of linking to HTML tables and text tables for read-only access. You can use and look at tables
in HTML or text format; however, the tables can’t be updated and records can’t be added to them using
Access.
A big disadvantage of working with linked tables is that you lose the capability to enforce referen-
tial integrity between tables (unless all the linked tables are in the same external Access database,
or all are in some other database management system that supports referential integrity). Linked
tables may exhibit somewhat poorer performance than local tables. Depending on the source, and
the location of the source data, users might experience a noticeable delay when they open a form
or report that is based on linked data.Performance issues become more pronounced when joining linked and local data in a query.
Because Access is unable to apply optimization techniques to foreign data, many joins are ineffi-
cient and require a lot of memory and CPU time to complete. However, Access’s outstanding abil-
ity to work with so many different types of external data makes it the ideal platform for
applications requiring these features.When to import external data
Importing data enables you to bring an external table or data source into a new or existing Access
table. By importing data, Access automatically converts data from the external format and copies it
into Access. You can even import data objects into a different Access database or Access project
than the one that is currently open. If you know that you’ll use your data in Access only, you
should import it. Generally, Access works faster with its own local tables.Note
Because importing makes another copy of the data, you might want to delete the old file after you import the
copy into Access. Sometimes, however, you’ll want to preserve the old data file. For example, the data might
be an Excel spreadsheet still in use. In cases such as this, simply maintain the duplicate data and accept that
storing it will require more disk space (and that the two files are going to get out of sync).
One of the principal reasons to import data is to customize it to meet your needs. After a table has
been imported into an Access database, you can work with the new table as if you’d built it in the
current database. With linked tables, on the other hand, you’re greatly limited in the changes you
can make. For example, you can’t specify a primary key or assign a data-entry rule, which means
that you can’t enforce integrity against the linked table. Also, because linked tables point to exter-
nal files, which Access expects to find in a specific location, it can make distributing your applica-
tion more difficult.Data is frequently imported into an Access database from an obsolete system being replaced by a
new Access application. When the import process is complete, the obsolete application can be
removed from the user’s computer.