Understanding relationship

There are three types of table relationships in Access. A customer can place any number of orders. It follows that for any customer represented in the Customers table, there might be many orders represented in the Orders table.

Understanding relationship

There are three types of table relationships in Access.

Understanding relationship

A customer can place any number of orders. It follows that for any customer represented in the Customers table, there might be many orders represented in the Orders table. The relationship between the Customers table and the Orders table is a one-to-many relationship.

To represent a one-to-many relationship in your database design, take the primary key on the "one" side of the relationship and add it as an additional field or fields to the table on the "many" side of the relationship. Access can then use the Customer ID number in the Orders table to locate the correct customer for each order.

A single order can include more than one product. On the other hand, a single product can appear on many orders. Therefore, for each record in the Orders table, there can be many records in the Products table. In addition, for each record in the Products table, there can be many records in the Orders table.

This relationship is called a many-to-many relationship. Note that to detect existing many-to-many relationships between your tables, it is important that you consider both sides of the relationship. To represent a many-to-many relationship, you must create a third table, often called a junction table, that breaks down the many-to-many relationship into two one-to-many relationships.

You insert the primary key from each of the two tables into the third table. As a result, the third table records each occurrence, or instance, of the relationship.

For example, the Orders table and the Products table have a many-to-many relationship that is defined by creating two one-to-many relationships to the Order Details table.

One order can have many products, and each product can appear on many orders. A one-to-one relationship In a one-to-one relationship, each record in the first table can have only one matching record in the second table, and each record in the second table can have only one matching record in the first table.

This relationship is not common because, most often, the information related in this way is stored in the same table.

You might use a one-to-one relationship to divide a table with many fields, to isolate part of a table for security reasons, or to store information that applies only to a subset of the main table.

When you do identify such a relationship, both tables must share a common field. Top of Page Why create table relationships? You can create table relationships explicitly by using the Relationships window, or by dragging a field from the Field List pane.

Understanding relationship

Access uses table relationships to decide how to join tables when you need to use them in a database object. There are several reasons why you should create table relationships before you create other database objects, such as forms, queries and reports. Table relationships inform your query designs To work with records from more than one table, you often must create a query that joins the tables.

The query works by matching the values in the primary key field of the first table with a foreign key field in the second table. For example, to return rows that list all of the orders for each customer, you construct a query that joins the Customers table with the Orders table based on the Customer ID field.

In the Relationships window, you can manually specify the fields to join. But, if you already have a relationship defined between the tables, Access supplies the default join, based on the existing table relationship.

In addition, if you use one of the query wizards, Access uses the information it gathers from the table relationships you have already defined to present you with informed choices and to prepopulate property settings with appropriate default values.

Table relationships inform your form and report designs When you design a form or report, Access uses the information it gathers from the table relationships you have already defined to present you with informed choices and to prepopulate property settings with appropriate default values.

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Table relationships are the foundation upon which you can enforce referential integrity to help prevent orphan records in your database.Guide to table relationships. Understanding referential integrity. View table relationships.

Create a table relationship. To create a one-to-one relationship Both of the common fields (usually the primary key and foreign key fields) must have a unique index.

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