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In general, tables can be related in one of three different ways: one-to-one, one- to-many or many-to-many. The relationship is used to cross reference. Relational data is like that puzzle, except for one thing: Access Access assigns the default one-to-many relationship between the two tables. In this post we are going to take a look at the One to Many Relationship in Access We will go through the process of setting up a One to.
You may well be familiar with the term 'Relational Database' already, but perhaps don't understand what it means.
Create a relationship
Relational Database design is all about how database tables are connected with each other. Whilst the novice database designer will create one single table with lots of fields, the relational database designer will look at the the data that needs to be stored and will put it together in a number of groups. Each group is then made into a separate table.
But because the subjects of the data stored in separate tables are connected in the real world - for example customers and orders - we need to create a relationship between the tables within the database itself. Lets take a look at how we go about doing this in practice. Imagine we have a number of customers who order electronic products by email, and we want to create a database to record both the customer's details and their orders.
Microsoft Access Tips: The One to Many Relationship
We do this for the simple reason that if a given customer makes more than one order, we only have to enter the customer details once in the customer table. Because we are going to create a relationship between the customer table and the orders table, all orders for individual customers will be connected with the correct persons' customer details.
This is made possible by connecting the primary key field of the customer table with the corresponding foreign key field in the orders table. If you remember, Primary and Foreign Keys were covered in my last blog post.
Create a one-to-many relationship in Access
This will open the Access relationships screen. Access will ask you which tables you want to show on the relationship screen. Select the two tables you want to create the one-to-many relationship for and click Add. The tables will then appear on the relationship screen.
Drag and drop the primary key of Customer to the soon to be foreign key in the Order table. If you click Create right now you will have created a one-to-many relationship. The Enforce Referential Integrity option If you select the Enforce Referential Integrity option Access will make sure that each record in the Order table refers to an existing record in the Customer table.
Selecting this option makes it impossible to create Order records thar refer to an non-existent customer. You should select Enforce Referential Integrity by default, because it protects the integrity of your data. In case of our one-to-many example this means that if the primary key of a customer one changes, Access will automatically update the foreign keys that refer to this customer in the Order table many.
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.
Many-to-many relationships can not be directly represented in relational database programs and have to be built by using two or more one-to-many relationships.
Defining relationships You define a relationship by adding the tables that you want to relate to the Relationships window, and then dragging the key field from one table and dropping it on the key field in the other table.