Guide to table relationships - Access
Applies to: Access Access Microsoft Office Access How to define a one-to-many or one-to-one relationship; How to define a. A well-organized Access database has relationships between fields in different tables. If you haven't yet defined any relationships, the Show Table dialog box To create a one-to-many relationship The field on the one side (typically the. going to take a look at the One to Many Relationship in Access this should put you on the right track to penetrating the meaning of.
Select the field or fields that you want to define as the primary key. To select one field, click the row selector for the desired field.
To select multiple fields, hold down the Ctrl key, and then click the row selector for each field. In Access or in Accessclick Primary Key on the toolbar. Define a one-to-many relationship between each primary table and the junction table.
Referential integrity Referential integrity is a system of rules that Access uses to make sure that relationships between records in related tables are valid, and that you do not accidentally delete or change related data. The matching field from the primary table is a primary key or has a unique index. The related fields have the same data type. There are two exceptions. Both tables belong to the same Access database.
Create a relationship - Access
If the tables are linked tables, they must be tables in Access format, and you must open the database in which they are stored to set referential integrity. Referential integrity cannot be enforced for linked tables from databases in other formats. The following rules apply when you use referential integrity: You cannot enter a value in the foreign key field of the related table that does not exist in the primary key of the primary table.
However, you can enter a Null value in the foreign key. This specifies that the records are unrelated.
For example, you cannot have an order that is assigned to a customer who does not exist. You cannot delete a record from a primary table if matching records exist in a related table.Microsoft Access Relationship Types
For example, you cannot delete an employee record from the "Employees" table if there are orders assigned to the employee in the "Orders" table. You cannot change a primary key value in the primary table if that record has related records. For example, you cannot change an employee's ID in the "Employees" table if there are orders assigned to that employee in the "Orders" table. Cascading updates and deletes For relationships in which referential integrity is enforced, you can specify whether you want Access to automatically cascade update or cascade delete related records.
If you set these options, delete and update operations that would usually be prevented by referential integrity rules are enabled. When you delete records or change primary key values in a primary table, Access makes the necessary changes to related tables to preserve referential integrity.
Access cascades updates without displaying any message. 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 that are related to the "Orders" records. However, when you delete records by using a delete query, Access automatically deletes the records in related tables without displaying a warning.
- Guide to table relationships
- Create a one-to-many relationship in Access
- How to define relationships between tables in an Access database
Join types There are three join types. You can see them in the following screen shot: Option 1 defines an inner join. An inner join is a join in which 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.
Follow these steps to create the one-to-many relationship Create two tables. The many-table Order in this example must have a field that will become the foreign key.
This field must have the same data type as the primary key it will refer to the primary key of Customer in this example. You can choose any name for the field. The name of a foreign key field doesn't have to be the same as the primary key field it refers to, but it is allowed. Select the Database Tools tab on the ribbon and then click the Relationships button. This will open the Access relationships screen.
Access will ask you which tables you want to show on the relationship screen. Select the two tables you want to create the one-to-many relationship for and click Add. The tables will then appear on the relationship screen. The first step is to create a new Access Database.
Lets call it Customer Orders. CustomerId is going to be the foreign field when we create the relationship. We are now going to select which tables are going to be used in the relationship. There are just two tables in our example database. Click on tblCustomer to highlight it in blue if it is not highlighted already. You will notice in the diagram below that each box has it's fields listed and primary keys indicated.
Notice the mouse pointer image changes from a circle with a line across to a plus sign as it hovers over the fields in tblOrder. We won't be going into what these do in this blog post. Our table relationship is now in place. This is what the database window looks like now.