MS Access Create Relationships
In Access, it is common to define a Primary Key field in each table, The term Foreign Key (FK) in a Relational Database System refers to a. inclusion of this field in the second table is called a “foreign key”. After the tables are created, relationships are defined by linking the related data together. You can create a table relationship by using the field is an AutoNumber field, however, the foreign key field.
You cannot create or change relationships between open tables. In Access or Accessfollow these steps: Press F11 to switch to the Database window. On the Tools menu, click Relationships.
If you have not yet defined any relationships in your database, the Show Table dialog box is automatically displayed. To create a relationship between a table and itself, add that table two times. Drag the field that you want to relate from one table to the related field in the other table.
To drag multiple fields, press Ctrl, click each field, and then drag them. In most cases, you drag the primary key field this field is displayed in bold text from one table to a similar field this field frequently has the same name that is called the foreign key in the other table.
Make sure that the field names that are displayed in the two columns are correct. You can change the names if it is necessary. 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. For example, you cannot delete an employee record from the "Employees" table if there are orders assigned to the employee in the "Orders" table.
Guide to table relationships
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, to represent a one-to-many relationship you take the primary key from the "one" table and add it as an additional field to the "many" table. To bring the data back together, Access takes the value in the "many" table and looks up the corresponding value in the "one" table.
In this way the values in the "many" table reference the corresponding values in the "one" table.
Suppose you have a one-to-many relationship between Shippers and Orders and you want to delete a Shipper. If the shipper you want to delete has orders in the Orders table, those orders will become "orphans" when you delete the Shipper record. The orders will still contain a shipper ID, but the ID will no longer be valid, because the record that it references no longer exists.
The purpose of referential integrity is to prevent orphans and keep references in sync so that this hypothetical situation never occurs.Primary Key Foreign Key Tutorial
You enforce referential integrity by enabling it for a table relationship see Enforce referential integrity for step-by-step instructions. Once enforced, Access rejects any operation that violates referential integrity for that table relationship.
This means Access will reject both updates that change the target of a reference, and deletions that remove the target of a reference. For such cases, what you really need is for Access to automatically update all the effected rows as part of a single operation.
That way, Access ensures that the update is completed in full so that your database is not left in an inconsistent state, with some rows updated and some not. When you enforce referential integrity and choose the Cascade Update Related Fields option, and you then update a primary key, Access automatically updates all fields that reference the primary key. When you enforce referential integrity and choose the Cascade Delete Related Records option, and you then delete a record on the primary key side of the relationship, Access automatically deletes all records that reference the primary key.
The Relationships window opens and displays any existing relationships. If no table relationships have been defined and you are opening the Relationships window for the first time, Access prompts you to add a table or query to the window. Open the Relationships window Click File, and then click Open. Select and open the database. On the Database Tools tab, in the Relationships group, click Relationships. If the database contains relationships, the Relationships window appears.
If the database does not contain any relationships and you are opening the Relationships window for the first time, the Show Table dialog box appears. Click Close to close the dialog box. On the Design tab, in the Relationships group, click All Relationships. This displays all of the defined relationships in your database. Note that hidden tables tables for which the Hidden check box in the table's Properties dialog box is selected and their relationships will not be shown unless the Show Hidden Objects check box is selected in the Navigation Options dialog box.
A table relationship is represented by a relationship line drawn between tables in the Relationships window. A relationship that does not enforce referential integrity appears as a thin line between the common fields supporting the relationship. When you select the relationship by clicking its line, the line thickens to indicate it is selected.
If you enforce referential integrity for this relationship, the line appears thicker at each end. When the Relationships window is active, you can select from the following commands on the ribbon: On the Design tab, in the Tools group: When you select a relationship line, you can click Edit Relationships to change the table relationship.
You can also double-click the relationship line. The report shows only the tables and relationships that are not hidden in the Relationships window.
On the Design tab, in the Relationships group: Note that hidden tables tables for which the Hidden check box in the table's Properties dialog box is selected and their relationships will not be shown unless Show Hidden Objects is selected in the Navigation Options dialog box. If you made any changes to the layout of the Relationships window, you are asked whether to save those changes.
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.
Create a relationship - Access
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.
MS Access - Create Relationships
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.