A primary feature of relational databases is that they contain multiple tables, each of which can have Screenshot of creating a relationship in MS Access Database Relationships in Microsoft Access The power of a database makes it possible to correlate data in many ways and ensure the. Describes how to define relationships in a database in Access , Access This article applies only to a Microsoft Access database .mdb guiadeayuntamientos.info). or one-to-one relationship; How to define a many-to-many relationship.
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.
Create a relationship - Access
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. 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.
- How to define relationships between tables in an Access database
- Guide to table relationships
- Create a relationship
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. Kinds of table relationships A relationship works by matching data in key columns, usually columns or fields that have the same name in both tables. In most cases, the relationship connects the primary key, or the unique identifier column for each row, from one table to a field in another table. The column in the other table is known as the "foreign key. There are three kinds of relationships between tables.
The kind of relationship that is created depends on how the related columns are defined. One-to-many relationships A one-to-many relationship is the most common kind of relationship. In this kind of relationship, a row in table A can have many matching rows in table B.
But a row in table B can have only one matching row in table A. For example, the "Publishers" and "Titles" tables have a one-to-many relationship. That is, each publisher produces many titles. But each title comes from only one publisher. A one-to-many relationship is created if only one of the related columns is a primary key or has a unique constraint. In the relationship window in Access, the primary key side of a one-to-many relationship is denoted by a number 1.
The foreign key side of a relationship is denoted by an infinity symbol. Many-to-many relationships In a many-to-many relationship, a row in table A can have many matching rows in table B, and vice versa. You create such a relationship by defining a third table that is called a junction table. The primary key of the junction table consists of the foreign keys from both table A and table B.
For example, the "Authors" table and the "Titles" table have a many-to-many relationship that is defined by a one-to-many relationship from each of these tables to the "TitleAuthors" table. One-to-one relationships In a one-to-one relationship, a row in table A can have no more than one matching row in table B, and vice versa. A one-to-one relationship is created if both of the related columns are primary keys or have unique constraints.
This kind of relationship is not common, because most information that is related in this manner would be in one table. You might use a one-to-one relationship to take the following actions: Divide a table with many columns.
Isolate part of a table for security reasons. Store data that is short-lived and could be easily deleted by deleting the table. Store information that applies only to a subset of the main table. In Access, the primary key side of a one-to-one relationship is denoted by a key symbol. The foreign key side is also denoted by a key symbol.
How to define relationships between tables When you create a relationship between tables, the related fields do not have to have the same names. However, related fields must have the same data type unless the primary key field is an AutoNumber field. You can match an AutoNumber field with a Number field only if the FieldSize property of both of the matching fields is the same. Even when both matching fields are Number fields, they must have the same FieldSize property setting.
How to define a one-to-many or one-to-one relationship To create a one-to-many or a one-to-one relationship, follow these steps: You cannot create or change relationships between open tables.
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. 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. 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.