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Novice: Requires knowledge of the user interface on single-user computers.
This article applies only to a Microsoft Access database (.mdb).
This article describes how to define relationships in a Microsoft Access database. It includes the following topics:
What Are Table RelationshipsIn a relational database, relationships enable you to prevent redundant data. For example, if you are designing a database that will track information about books, you might have a table called Titles that stores information about each book, such as the book?s title, date of publication, and publisher. There is also information you might want to store about the publisher, such as the publisher's phone number, address, and zip code. If you were to store all of this information in the titles table, the publisher?s phone number would be duplicated for each title that the publisher prints.
A better solution is to store the publisher information only once in a separate table, Publishers. You would then put a pointer in the Titles table that references an entry in the Publishers table.
To make sure that your data is not out of sync, you can enforce referential integrity between the Titles and Publishers tables. Referential integrity relationships help ensure that information in one table matches information in another. For example, each title in the Titles table must be associated with a specific publisher in the Publishers table. A title cannot be added to the database for a publisher that does not exist in the database.
Types of Table RelationshipsA relationship works by matching data in key columns, usually columns with the same name in both tables. In most cases, the relationship matches the primary key from one table, which provides a unique identifier for each row, with an entry in the foreign key in the other table. For example, sales can be associated with the specific titles sold by creating a relationship between the title_id column in the Titles table (the primary key) and the title_id column in the Sales table (the foreign key).
There are three types of relationships between tables. The type of relationship that is created depends on how the related columns are defined.
One-To-Many RelationshipsA one-to-many relationship is the most common type of relationship. In this type of relationship, a row in table A can have many matching rows in table B, but a row in table B can have only one matching row in table A. For example, the Publishers and Titles tables have a one-to-many relationship: each publisher produces many titles, but each title comes from only one publisher.
A one-to-many relationship is created if only one of the related columns is a primary key or has a unique constraint.
In Access, the primary key side of a one-to-many relationship is denoted by a key symbol. The foreign key side of a relationship is denoted by an infinity symbol.
Many-To-Many RelationshipsIn a many-to-many relationship, a row in table A can have many matching rows in table B, and vice versa. You create such a relationship by defining a third table, called a junction table, whose primary key consists of the foreign keys from both table A and table B. For example, the Authors table and the Titles table have a many-to-many relationship that is defined by a one-to-many relationship from each of these tables to the TitleAuthors table. The primary key of the TitleAuthors table is the combination of the au_id column (the authors table?s primary key) and the title_id column (the Titles table?s primary key).
One-To-One RelationshipsIn a one-to-one relationship, a row in table A can have no more than one matching row in table B, and vice versa. A one-to-one relationship is created if both of the related columns are primary keys or have unique constraints.
This type of relationship is not common because most information related in this way would be all in one table. You might use a one-to-one relationship to:
How to Define Relationships Between TablesWhen you create a relationship between tables, the related fields do not have to have the same names. However, related fields must have the same data type unless the primary key field is an AutoNumber field. You can match an AutoNumber field with a Number field only if the FieldSize property of both of the matching fields is the same. For example, you can match an AutoNumber field and a Number field if the FieldSize property of both fields is Long Integer. Even when both matching fields are Number fields, they must have the same FieldSize property setting.
How to Define a One-To-Many or One-To-One RelationshipsTo create a one-to-many or a one-to-one relationship, follow these steps:
How to Define a Many-To-Many RelationshipsTo create a many-to-many relationship, follow these steps:
Referential IntegrityReferential integrity is a system of rules that Microsoft Access uses to ensure that relationships between records in related tables are valid, and that you do not accidentally delete or change related data. You can set referential integrity when all of the following conditions are met:
Cascading Updates and DeletesFor relationships in which referential integrity is enforced, you can specify whether you want Microsoft Access to automatically cascade update or cascade delete related records. If you set these options, delete and update operations that would normally be prevented by referential integrity rules are allowed. When you delete records or change primary key values in a primary table, Microsoft Access makes the necessary changes to related tables to preserve referential integrity.
If you click to select the Cascade Update Related Fields check box when you define a relationship, any time that you change the primary key of a record in the primary table, Microsoft Access automatically updates the primary key to the new value in all related records. For example, if you change a customer's ID in the Customers table, the CustomerID field in the Orders table is automatically updated for every one of that customer's orders so that the relationship is not broken. Microsoft Access cascades updates without displaying any message.
NOTE: If the primary key in the primary table is an AutoNumber field, selecting the Cascade Update Related Fields check box will have no effect, because you cannot change the value in an AutoNumber field.
If you select the Cascade Delete Related Records check box when you define a relationship, any time that you delete records in the primary table, Microsoft Access automatically deletes related records in the related table. For example, if you delete a customer record from the Customers table, all the customer's orders are automatically deleted from the Orders table (this includes records in the Order Details table related to the Orders records). When you delete records from a form or datasheet with the Cascade Delete Related Records check box selected, Microsoft Access warns you that related records may also be deleted. However, when you delete records using a delete query, Microsoft Access automatically deletes the records in related tables without displaying a warning.
Join TypesThere are three join types, as follows:
Option 1 defines an inner join. An inner join is a join where records from two tables are combined in a query's results only if values in the joined fields meet a specified condition. In a query, the default join is an inner join that selects records only if values in the joined fields match.
Option 2 defines a left outer join. A left outer join is a join in which all the records from the left side of the LEFT JOIN operation in the query's SQL statement are added to the query's results, even if there are no matching values in the joined field from the table on the right.
Option 3 defines a right outer join. A right outer join is a join in which all the records from the right side of the RIGHT JOIN operation in the query's SQL statement are added to the query's results, even if there are no matching values in the joined field from the table on the left.
For additional information about relationships in Microsoft Access, click the "Creating and Designing Tables" topic on the Microsoft Access 2000 Help page at the following Microsoft Web site:
Microsoft Access 2000 HelpFor more information about relationships in Microsoft Access, click Microsoft Access Help on the Help menu, type Create or Modify Relationships in the Office Assistant or the Answer Wizard, and then click Search to view the topics returned.
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