Guide to table relationships - Access
Calculated Field Properties (Access only). 13 Importing data from Excel. 14 . Ensure all the data in your database is relevant and kept up-to-date. MS Access One-To-One Relationship - Learn MS Access in simple and easy This relationship is used to relate one record from one table to one and only one Hire Date. Click on the Save icon and open tblHRData and you will see that the . how to define relationships in a database in Access , Access , This article applies only to a Microsoft Access database .mdb or.
Set the relationship options if it is necessary. These options will be explained in detail later in this article. Click Create to create the relationship. Repeat steps 4 through 7 for each pair of tables that you want to relate. Whether you save the layout or do not save the layout, the relationships that you create are saved in the database.
However, referential integrity is not enforced with queries. How to define a many-to-many relationship To create a many-to-many relationship, follow these steps: Create the two tables that will have a many-to-many relationship. Create a third table. This is the junction table. In the junction table, add new fields that have the same definitions as the primary key fields from each table that you created in step 1.
In the junction table, the primary key fields function as foreign keys. You can add other fields to the junction table, just as you can to any other table. In the junction table, set the primary key to include the primary key fields from the other two tables.
Note To create a primary key, follow these steps: Open a table in Design view. 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. 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.
How to define relationships between tables in an Access database
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.
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. 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 side.
Option 3 defines a right outer join. Top of Page Create a table relationship You can create a table relationship by using the Relationships window, or by dragging a field onto a datasheet from the Field List pane. When you create a relationship between tables, the common fields are not required to have the same names, although it is often the case that they do. Rather, those fields must have the same data type.
If the primary key field is an AutoNumber field, however, the foreign key field can be a Number field if the FieldSize property of both fields is the same.
When both common fields are Number fields, they must have the same FieldSize property setting.
Creating Primary Keys and Foreign Keys in Microsoft Access | MS Office User
Create a table relationship by using the Relationships window Click File, and then click Open. If you have not yet defined any relationships, the Show Table dialog box automatically appears. If it does not appear, on the Design tab, in the Relationships group, click Show Table. The Show Table dialog box displays all of the tables and queries in the database. To see only tables, click Tables. To see only queries, click Queries. To see both tables and queries, click Both.
Select one or more tables or queries and then click Add. When you have finished adding tables and queries to the Relationships window, click Close. Drag a field typically the primary key from one table to the common field the foreign key in the other table. To drag multiple fields, press the CTRL key, click each field, and then drag them.
The Edit Relationships dialog box appears. Verify that the field names shown are the common fields for the relationship. If a field name is incorrect, click the field name and select a new field from the list. To enforce referential integrity for this relationship, select the Enforce Referential Integrity check box. For more information about referential integrity, see the Understanding Referential Integrity and the Enforce Referential Integrity sections.
The relationship line is drawn between the two tables. If you selected the Enforce Referential Integrity check box, the line appears thicker at each end. This means the Indexed property for these fields should be set to Yes No Duplicates. If both fields have a unique index, Access creates a one-to-one relationship. This means the Indexed property for this field should be set to Yes No Duplicates.
The field on the "many" side should not have a unique index. When one field has a unique index and the other does not, Access creates a one-to-many relationship. Create a table relationship by using the Field List pane You can add a field to an existing table that is open in Datasheet view by dragging it from the Field List pane. The Field List pane shows fields available in related tables and also fields available in other tables.
When you drag a field from an "other" unrelated table and then complete the Lookup Wizard, a new one-to-many relationship is automatically created between the table in the Field List pane and the table to which you dragged the field.
This relationship, created by Access, does not enforce referential integrity by default. To enforce referential integrity, you must edit the relationship. See the section Change a table relationship for more information. Open a table in Datasheet view On the File tab, click Open. In the Open dialog box, select and open the database. In the Navigation Pane, right-click the table to which you want to add the field and create the relationship, and then click Open.
The Field List pane appears. The Field List pane shows all of the other tables in your database, grouped into categories. When you work with a table in Datasheet view, Access displays fields in either of two categories in the Field List pane: Fields available in related tables and Fields available in other tables.
The first category lists all of the tables that have a relationship with the table you are currently working with. The second category lists all of the tables with which your table does not have a relationship. To add a field to your table, drag the field that you want from the Field List pane to the table in Datasheet view. Drag the field that you want from the Field List pane to the table that is open in Datasheet view.
When the insertion line appears, drop the field in position. The Lookup Wizard starts. Follow the instructions to complete the Lookup Wizard. The field appears in the table in Datasheet view. When you drag a field from an "other" unrelated table and then complete the Lookup Wizard, a new one-to-many relationship is automatically created between the table in the Field List and the table to which you dragged the field.
Top of Page Delete a table relationship To remove a table relationship, you must delete the relationship line in the Relationships window. Carefully position the cursor so that it points at the relationship line, and then click the line. The relationship line appears thicker when it is selected. Note that when you remove a relationship, you also remove referential integrity support for that relationship, if it is enabled. As a result, Access will no longer automatically prevent the creation of orphan records on the "many" side of a relationship.
The Relationships window appears. If you have not yet defined any relationships and this is the first time you are opening the Relationships window, the Show Table dialog box appears. If the dialog box appears, click Close. All tables that have relationships are displayed, showing relationship lines. Click the relationship line for the relationship that you want to delete. Access might display the message Are you sure you want to permanently delete the selected relationship from your database?.
If this confirmation message appears, click Yes. If either of the tables employed in the table relationship are in use, perhaps by another person or process, or in an open database object such as a formyou will not be able to delete the relationship.
You must first close any open objects that use these tables before you can remove the relationship. Top of Page Change a table relationship You change a table relationship by selecting it in the Relationships window and then editing it.
Carefully position the cursor so that it points at the relationship line, and then click the line to select it. With the relationship line selected, double-click it or click Edit Relationships in the Tools group on the Design tab. Click the relationship line for the relationship that you want to change. Double-click the relationship line. Make your changes, and then click OK. The Edit Relationships dialog box allows you to change a table relationship.
Specifically, you can change the tables or queries on either side of the relationship, or the fields on either side. You can also set the join type, or enforce referential integrity and choose a cascade option. For more information about the join type and how to set it, see the section Set the join type.
For more information about how to enforce referential integrity and choose a cascade option, see the section Enforce referential integrity. Set the join type When you define a table relationship, the facts about the relationship inform your query designs. For example, if you define a relationship between two tables, and you then create a query that employs those tables, Access automatically selects the default matching fields based upon the fields specified in the relationship.
You can override these initial default values in your query, but the values supplied by the relationship will often prove to be the correct ones. Because matching and bringing together data from more than one table is something you will do frequently in all but the most simple databases, setting defaults by creating relationships can be time saving and beneficial.
A multiple table query combines information from more than one table by matching the values in common fields. The operation that does the matching and combining is called a join. For example, suppose you want to display customer orders. The query result contains customer information and order information for only those rows where a corresponding match was found.
One of the values you can specify for each relationship is the join type. The join type tells Access which records to include in a query result.