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1992-10-12
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======================================================================
Microsoft(R) Product Support Services Application Note (Text File)
WX0635: DATABASE STRUCTURE QUESTIONS & ANSWERS
======================================================================
Revision Date: 10/92
No Disk Included
The following information applies to Microsoft Access(TM) version 1.0.
--------------------------------------------------------------------
| INFORMATION PROVIDED IN THIS DOCUMENT AND ANY SOFTWARE THAT MAY |
| ACCOMPANY THIS DOCUMENT (collectively referred to as an |
| Application Note) IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY |
| KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO |
| THE IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A |
| PARTICULAR PURPOSE. The user assumes the entire risk as to the |
| accuracy and the use of this Application Note. This Application |
| Note may be copied and distributed subject to the following |
| conditions: 1) All text must be copied without modification and |
| all pages must be included; 2) If software is included, all files |
| on the disk(s) must be copied without modification [the MS-DOS(R) |
| utility DISKCOPY is appropriate for this purpose]; 3) All |
| components of this Application Note must be distributed together; |
| and 4) This Application Note may not be distributed for profit. |
| |
| Copyright 1992 Microsoft Corporation. All Rights Reserved. |
| Microsoft and MS-DOS are registered trademarks and Microsoft |
| Access and Windows are trademarks of Microsoft Corporation. |
| 1-2-3 and Lotus are registered trademarks of Lotus Development |
| Corporation. Btrieve is a registered trademark of SoftCraft, Inc., |
| a Novell Company. dBASE III PLUS and dBASE IV are registered |
| trademarks of Ashton-Tate Corporation. Paradox is a registered |
| trademark of Ansa Software, a Borland company. |
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1. Q. What foreign data formats does Microsoft Access support?
A. Microsoft Access can link to Btrieve(R), dBASE III PLUS(R), dBASE
IV(R), and Paradox(R) versions 3.x data, including indexes.
Microsoft Access can import data from Btrieve, dBASE III PLUS,
dBASE IV, FoxPro versions 2.x, Lotus(R) 1-2-3(R) versions 2.x and
3.x, Lotus 1-2-3/W, Microsoft Excel for Windows versions 2.x and
later, Paradox 3.x, and fixed-length and delimited ASCII text.
Microsoft Access also supports the Open Database Connectivity
(ODBC) specification for connectivity to data on database
servers. ODBC drivers for Microsoft SQL Server are included with
Microsoft Access. Microsoft SQL Server ODBC drivers are the only
drivers certified for Microsoft Access version 1.0.
2. Q. What are primary and foreign keys in a relational database?
A. Primary keys uniquely identify a specific row in a table. They
are usually composed of a single field or column whose data is
unique to each row of information (for example, a customer
identification number, employee social security number, or order
identification number). Primary keys also can be composed of more
than one field. For example, the fields Company Identification
and Contact Name could be combined to form a primary key for a
table that contains information about companies that have more
than one contact.
Foreign keys identify rows in one table that match the primary
key of another table. For example, if the field Employee ID is
the primary key in the Employee table, in the Orders table,
Employee ID is the foreign key. The relationship between primary
and foreign keys enables you to access related data in multiple
tables.
3. Q. Which FieldSize setting should I select for the Number data
type in order to use a Counter field of another table as the
foreign key in the current table?
A. To use a Counter field of another table as the foreign key in the
current table, select the Long Integer setting.
4. Q. How do you change the starting value of a Counter column to
a number other than 1?
A. To set the starting value of a Counter column to a number other
than 1, use the following four-step procedure:
1. Create an additional single-column temporary table. Give
the Number column the same name as the Counter column in the
original table.
2. Create a new row in the temporary table by inserting a
value in the Number column. This value should be 1 less than
the desired starting value for the original table.
3. Create an Append query to append the single row from the
temporary table to the original table.
4. Once you append the single row, delete the temporary
table. You can then delete the dummy row from the original
table.
NOTE: Do not compact the database before you add the first row
to the original table. If you do this, the counter value is
reset to start at 1.
5. Q. Is data in tables stored in a sorted order? How can I view
my data in sorted order?
A. Data in tables is not stored in sorted order. Data is stored in
the order in which it was entered. To view data in sorted order,
create either a query, or a form based on a query, that uses the
Sort field on the query grid. By default, if the table includes a
primary key, the Datasheet view of the table will display the
data sorted by the primary key. To view the data sorted by
something other than the primary key alone, use a query, as
previously described.
6. Q. How can I delete the primary key from a table?
A. You can delete the primary key for a table by opening the Table
Properties dialog box and deleting the entry for the Primary Key
field. Please note that the primary key cannot be deleted from a
table that is the primary table in a relationship.
7. Q. How can I create computed columns in tables?
A. You can create computed columns or expressions with queries.
Within the query, create a column that is defined as an
expression. In general, it is helpful to think of queries as
virtual tables; you can use a query wherever you can use a table.
If you're familiar with SQL terminology, this is very similar to
creating a view. Unlike most implementations of views, however,
Microsoft Access views can be updated even if they involve joins
from different data sources, such as Paradox, Btrieve, or
separate Microsoft Access data sources.