Guide to table relationships - Access
A one-to-one relationship is a link between the information in two tables, where one-to-one relationships by linking the index (usually the primary key) in one. In a one-to-one relationship, one record in a table is associated with one and is the primary key; in the Contact Info table, the Student ID field is a foreign key. Learn how to create a one-to-many relationship in Access in this step by step instruction. The customer_id field in the Order table is called a foreign 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. 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. 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.
- Create a one-to-many relationship in Access
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- Create one-to-one relationships
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.
Video: Create one-to-one relationships - Access
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 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. In a relational database Accessthe data in one table is related to the data in other tables. In general, tables can be related in one of three different ways: The relationship is used to cross reference information between tables.
One-to-One, One-to-Many Table Relationships in SQL Server
One to One In a one-to-one relationship each record in one table has at most one related record in another table. In a one-to-one relationship, each record in Table A can have only one matching record in Table B, and each record in Table B can have only one matching record in Table A. This type of relationship is not common, because most information related in this way would be in one 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.
One to Many A one-to-many relationship, often referred to as a "master-detail" or "parent-child" relationship. A one-to-many relationship is the most common type of relationship. In a one-to-many relationship, a record in Table A can have many matching records in Table B, but a record in Table B has only one matching record in Table A.
A many-to-many relationship is really two one-to-many relationships with a third table. A many-to-many relationship means that for each record in one table there can be many records in another table and for each record in the second table there can be many in the first.