MS Access One-To-One Relationship
MS Access One-To-One Relationship - Learn MS Access in simple and easy steps starting from basic to advanced concepts with examples including Overview. These primary key fields will later be used to create a many-to-many relationship between these tables. If you want to know how to create a table in design view. Describes how to define relationships in a database in Access , Access , Access , Access , or Access This article applies only to a Microsoft Access database .mdb socialgamenews.info). . When you create a relationship between tables, the related fields do not have to have the same.
However, setting up an outer join shows all the records in the Companies table, including those in which there is no corresponding record in the Contacts table. NOTE An outer join is also called a left-outer join. To see why, consider a one-to-many relation. Here, the "left" side is the "one," table and the "right" side is the "many" table. So this type of join includes every record from the "one" left side and only those matching records from the "many" right side.
You use the term left-outer join when you need to differentiate it from a right-outer join. In a one-to-many relation, this type of join includes every record from the "many" right side and only those matching records from the "one" left side. Self-joins are handy for tables that include different fields with the same type of information. For example, the Northwind Employees table has an EmployeeID field that lists the identification number of each employee.
The same table also includes a ReportsTo field that lists the identification number of the employee's manager. To display the name of each employee's manager, you use a second copy of the Employees table and join the EmployeeID and ReportsTo fields. For example, suppose you want to compare the unit price data in Northwind's Order Details table with the unit price data in the Products table. Specifically, you want to see those orders where the unit price of the order differs from the unit price of the product.
In this case, you look for records where the [Order Details]. UnitPrice field is not equal to the [Products]. Adding Tables to the Relationships Window If you need to establish a new relationship between two tables, your first order of business is to add the tables to the Relationships window.
Create a many-to-many relationship in Access
Here are the steps to follow: Access displays the Show Table dialog box, shown in Figure 3. Click the table you want to add. Access adds the table to the Relationships window. Repeat steps 2 and 3 to add more tables. Click Close to return to the Relationships window. Joining Tables To create a join between two tables or a self-join between two copies of the same tableuse the mouse to click and drag one of the related fields and drop it on the other. Here are the specific steps: Add the tables you want to join.
Arrange the table boxes so that in each box you can see the fields you want to use for the join. Click and drag the related field from one table and drop it on the related field in the other table.
- Establishing Table Relationships
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Access displays the Edit Relationships dialog box, shown in Figure 3. The grid should show the names of the fields in each table that you want to relate.
MS Access - One-To-One Relationship
If not, use the drop-down list in one or both cells to click the correct field or fields. 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. 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.
Create, edit or delete a relationship
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.