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Feb
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Warning: date() [function.date]: It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/New_York' for 'EST/-5.0/no DST' instead in /home/frodr/public_html/msofficetuneup.com/wp-includes/functions.php on line 107

Warning: date() [function.date]: It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/New_York' for 'EST/-5.0/no DST' instead in /home/frodr/public_html/msofficetuneup.com/wp-includes/functions.php on line 109

Warning: date() [function.date]: It is not safe to rely on the system's timezone settings. You are *required* to use the date.timezone setting or the date_default_timezone_set() function. In case you used any of those methods and you are still getting this warning, you most likely misspelled the timezone identifier. We selected 'America/New_York' for 'EST/-5.0/no DST' instead in /home/frodr/public_html/msofficetuneup.com/wp-includes/functions.php on line 111

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One-to-One Relationship in Access 2007

Microsoft Access Add comments

A one-to-one relationship links one record in a table to zero or one record in another table. People sometimes use one-to-one relationships to break down a table with lots of fields into two (or more) smaller tables.

A Products table may include detailed information that describes the product and its price, and additional information that describes how it’s built. This information’s important only to the people in the engineering department, so you may choose to split it into a separate table (named something like ProductsEngineering). That way, sales folks don’t need to think about it when they’re making an order. Other times, you might break a table into two pieces because it’s simply too big. (Access doesn’t let any table have more than 255 fields.)

You create a one-to-one relationship in the same way you create a one-to-many relationshipby dragging the fields in the Relationships tab (Figure 5-15). The only difference is that the linked fields in both tables need to be set to prevent duplicates. This way, a record in one table can (at most) be linked to a single record in the other table.

Note: A field prevents duplicates if it’s set as the primary key for a table (Section 2.4), or if it has an index that prevent duplicates (Section 4.1.3).

Figure 5-15. When you link two fields that don’t allow duplicates (and you have the Enable Referential Integrity option switched on), Access realizes that you’re creating a one-to-one relationship. Access places the number 1 at each side of the line to distinguish it from other types of relationships. In this example, the ID column in the Products table and the ID column in the ProductsEngineering table are both primary keys of their respective tables, so there’s no way to link more than one record in ProductsEngineering to the same record in Products.

WORD TO THE WISE
Approach One-to-One Relationships with Caution

One-to-one relationships are extremely rare in Access. Usually, features like column hiding (Section 3.1.4) and queries (Chapter 6) are better choices if you want to see only some of the fields in a table.

Splitting a table into two pieces complicates the design of your database, and you’d generally do it only if you have other reasons to separate the tables. Some possible examples include:

The two parts of the table need to be placed in separate databases so that different people can copy them to separate computers and edit them independently.
You want to stop prying eyes from seeing sensitive data. One way to do this is to put the information that should be secure into a separate table, and put that separate table in a different, more secure database file.
You have a table that stores huge amounts of data, like an Attachment field (Section 2.3.8) with large documents. In this case, you might get better performance by splitting the table. You might even choose to put one half of the table in a separate database.
Some of the data in your table’s optional. Rather than include a lot of blank fields, you can pop it into a separate table. If you don’t need to include this information, then you don’t need to add a record to the linked table.

If you don’t have these requirements, then you’re better off creating a single large table.

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Referential Integrity in Access 2007