Create a relationship - Access
In a relational database (Access), the data in one table is related to the data in other tables. In general, tables can be related in one of three different ways. To create a relationship in an Access web database, you use the. One of the advantages of Relational Database Management System such as Access is to easily setup tables and relationships with.
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.
Create a one-to-many relationship in Access
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.
Table relationships are the foundation upon which you can enforce referential integrity to help prevent orphan records in your database.
When you design a database, you divide your information into tables, each of which has a primary key. You then add foreign keys to related tables that reference those primary keys. These foreign key-primary key pairings form the basis for table relationships and multi-table queries. Referential integrity, which is dependent on table relationships, helps ensure that references stay synchronized.
Top of Page Understanding referential integrity When you design a database, you divide your database information into many subject-based tables to minimize data redundancy.
You then give Access a way to bring the data back together by placing common fields into related tables. 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.
Guide to table relationships
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.
- MS Access - One-To-One Relationship
- IT Services
- Create a relationship
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 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 you selected the Enforce Referential Integrity check box, the line appears thicker at each end. This means that 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 that 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. It can have an index, but it must allow duplicates. When one field has a unique index, and the other does not, Access creates a one-to-many relationship.
Top of Page Create a table relationship by using the Field List pane to add a field 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 in the database. 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 Edit a relationship for more information. Open a table in Datasheet view In the Navigation Pane, double-click the table.
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 with which you are currently working. 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 into 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.
Edit a relationship You change a relationship by selecting it in the Relationships window and then editing it. Carefully position the cursor so that it points to the relationship line, and then click the line to select it. The relationship line appears thicker when it is selected. Create a relationship in an Access web app The Relationships window isn't available in an Access web app.
Instead of creating a relationship in an Access web app, you create a lookup field that gets values from a related field in another table. The field that your lookup will use as the source for values must already exist before you create your lookup field. Open the table where you want to create a new lookup field by double-clicking it in the navigation. In the above example, click the Employees table. Click in the Field Name column just below the last field in the table and type a name for your new lookup field.
In the example, type Region as the field name. In the Data Type column, click the arrow and select Lookup. The Lookup Wizard starts.
MS Access One-To-One Relationship
On the first page of the Lookup Wizard, select I want the lookup field to get values from another table or query. More options appear in the dialog box. Select the name of the table or query that should provide the values for your lookup.
In the example, select Table: After you select the table, use the Which value do you want to display in your lookup list to select the field that you want to use as a display value for your lookup field.