Establishing Referential Integrity
As you can see, establishing a relationship is quite easy. Establishing the right kind of relationship is a little more difficult. When you attempt to establish a relationship between two tables, Access makes some decisions based on a few predefined factors:
Access establishes a one-to-many relationship if one of the related fields is a primary key or has a unique index. Access establishes a one-to-one relationship if both of the related fields are primary keys or have unique indexes. Access creates an indeterminate relationship if neither of the related fields is a primary key and neither has a unique index. You cannot establish referential integrity in this case.
As discussed earlier in this hour, referential integrity consists of a series of rules that the Jet Engine applies to ensure that Jet properly maintains the relationships between tables. At the most basic level, referential integrity rules prevent the creation of orphan records in the table on the "many" side of the one-to-many relationship. After you establish a relationship between a Customers table and an Orders table, for example, all orders in the Orders table must be related to a particular customer in the Customers table. Before you can establish referential integrity between two tables, the following conditions must be met:
The matching field on the "one" side of the relationship must be a primary key field or must have a unique index. The matching fields must have the same data types. (For linking purposes, AutoNumber fields match Long Integer fields.) With the exception of Text fields, the matching fields also must have the same size. Number fields on both sides of the relationship must have the same size (for example, Long Integer). Both tables must be part of the same Access database. Both tables must be stored in the proprietary Access file (.MDB) format. (They cannot be external tables from other sources.) The database that contains the two tables must be open. Existing data within the two tables cannot violate any referential integrity rules. All orders in the Orders table must relate to existing customers in the Customers table, for example.
 | Although Text fields involved in a relationship do not have to be the same size, it is prudent to make them the same size. Otherwise, you degrade performance as well as risk the chance of unpredictable results when you create queries based on the two tables. |
After you establish referential integrity between two tables, the Jet Engine applies the following rules:
You cannot enter in the foreign key of the related table a value that does not exist in the primary key of the primary table. For example, you cannot enter in the CustomerID field of the Orders table a value that does not exist in the CustomerID field of the Customers table. You cannot delete a record from the primary table if corresponding records exist in the related table. For example, you cannot delete a customer from the Customers table if related records (for example, records with the same value in the CustomerID field) exist in the Orders table. You cannot change the value of a primary key on the "one" side of a relationship if corresponding records exist in the related table. For example, you cannot change the value in the CustomerID field of the Customers table if corresponding orders exist in the Orders table.
If you attempt to violate any of these three rules and you have enforced referential integrity between the tables, Access displays an appropriate error message, as shown in Figure 9.7.
The Jet Engine's default behavior is to prohibit the deletion of parent records that have associated child records and to prohibit the change of a primary key value of a parent record when that parent has associated child records. You can override these restrictions by using the Cascade Update Related Fields and Cascade Delete Related Records check boxes that are available in the Relationships dialog box when you establish or modify a relationship.
The Cascade Update Related Fields Option
The Cascade Update Related Fields option is available only if you have established referential integrity between tables. When this option is selected, the user can change the primary key value of the record on the "one" side of the relationship. When the user attempts to modify the field joining the two tables on the "one" side of the relationship, the Jet Engine cascades the change down to the foreign key field on the "many" side of the relationship. This is useful if the primary key field is modifiable. For example, a purchase number on a purchase order master record might be updateable. If the user modifies the purchase order number of the parent record, you would want to cascade the change to the associated detail records in the purchase order detail table.
 | There is no need to select the Cascade Update Related Fields option when the related field on the "one" side of the relationship is an AutoNumber field. You can never modify an AutoNumber field. The Cascade Update Related Fields option has no effect on AutoNumber fields. In fact, this is why, in the preceding Task, you make the CustomerID a Text field. It provides an example that you can later use with a cascade update. |
It is very easy to accidentally introduce a loophole into a system. If you create a one-to-many relationship between two tables but forget to set the Required property of the foreign key field to Yes, you allow the addition of orphan records. Figure 9.8 illustrates this point. In this example, I added an order to tblOrders without entering a customer ID. This record is an orphan record because no records in tblCustomers have CustomerID set to Null. To eliminate the problem, you set the Required property of the foreign key field to Yes.
The Cascade Delete Related Records Option
The Cascade Delete Related Records option is available only if you have established referential integrity between tables. When this option is selected, the user can delete a record on the "one" side of a one-to-many relationship, even if related records exist in the table on the "many" side of the relationship. A user can delete a customer even if the customer has existing orders, for example. The Jet Engine maintains referential integrity between the tables because it automatically deletes all related records in the child table.
If you attempt to delete a record from the table on the "one" side of a one-to-many relationship and no related records exist in the table on the "many" side of the relationship, you get the usual warning message, as shown in Figure 9.9. On the other hand, if you attempt to delete a record from the table on the "one" side of a one-to-many relationship and related records exist in the child table, Access warns you that you are about to delete the record from the parent table as well as any related records in the child table (see Figure 9.10).
 | The Cascade Delete Related Records option is not always appropriate. It is an excellent feature, but you should use it prudently. Although it is usually appropriate to cascade delete from an Orders table to an Order Details table, for example, it generally is not appropriate to cascade delete from a Customers table to an Orders table. This is because you generally do not want to delete all your order history from the Orders table if for some reason you want to delete a customer. Deleting the order history causes important information, such as the profit and loss history, to change. It is therefore appropriate to prohibit this type of deletion and handle the customer in some other way, such as marking him or her as inactive or archiving his or her data. On the other hand, if you delete an order because the customer cancelled it, you probably want to remove the corresponding order detail information as well. In this case, the Cascade Delete Related Records option is appropriate. You need to make the most prudent decision in each situation, based on business needs. You need to carefully consider the implications of each option before you make a decision. |
Task: Working with Referential Integrity
 | In this task you enforce referential integrity between the tblCustomers table and the tblOrders table you created earlier in this hour. It illustrates how enforcing referential integrity between the tables affects the process of adding and deleting records. You need to follow these steps: |
Open the Relationships window. Double-click the join line between tblCustomers and tblOrders. Enable the Enforce Referential Integrity check box. Click OK. Repeat this process for the relationship between tblOrders and tblOrderDetails. Go into tblCustomer and add a couple records. Take note of the customer IDs. Go into tblOrders. Add a couple records, taking care to assign customer IDs of customers that exist in the tblCustomers table. Now try to add an order for a customer whose customer ID does not exist in tblCustomers. You should get an error message. Attempt to delete from tblCustomers a customer who does not have any orders. You should get a warning message, but Access should allow you to complete the process. Now try to delete a customer who does have orders. The Jet Engine should prohibit you from deleting the customer. Attempt to change the customer ID of a customer who has orders. You should not be able to do this.
Task: Working with Cascade Update Related Fields and Cascade Delete Related Records
| With the Cascade Update Related Fields feature enabled, you are able to update the primary key value of a record that has associated child records. With the Cascade Delete Related Records feature enabled, you can delete a parent record that has associated child records. To see how to use Cascade Update Related Fields and Cascade Delete Related Records, follow these steps: |
Modify the relationship between the tblCustomers and tblOrders tables you created earlier in this hour. Open the Relationships window. Double-click the join line between tblCustomers and tblOrders. Enable the Cascade Update Related Fields check box. Modify the relationship between tblOrders and tblOrderDetails. Enable the Cascade Delete Related Records check box. There is no need to enable Cascade Update Related Fields because the OrderID field in tblOrders is an AutoNumber field. Attempt to delete a customer who has orders. The Jet Engine should prohibit you from doing this because Cascade Delete Related Records is not enabled. In tblCustomers, change the customer ID of a customer who has orders. The Jet Engine should allow this change. Take a look at tblOrders. The Jet Engine should have updated the customer ID of all corresponding records in the table to reflect the change in the parent record. Add some order details to tblOrderDetails. Try to delete any order that has details within tblOrderDetails. You should receive a warning, but the Jet Engine should allow you to complete the process.
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