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Microsoft Access 2010 : Relating the Information in Your Database - The Types of Relationships

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4/20/2014 1:36:03 AM

Three types of relationships can exist between tables in a database: one to many, one to one, and many to many. Setting up the proper type of relationship between two tables in a database is imperative. The right type of relationship between two tables ensures

  • Data integrity

  • Optimal performance

  • Ease of use in designing system objects

The reasons behind these benefits are covered throughout this hour. Before you can understand the benefits of relationships, though, you must understand the types of relationships available.

One-to-Many Relationships

A one-to-many relationship is by far the most common type of relationship. In a one-to-many relationship, a record in one table can have many related records in another table. A common example is a relationship set up between a Customers table and an Orders table. For each customer in the Customers table, you want to have more than one order in the Orders table. On the other hand, each order in the Orders table can belong to only one customer. The Customers table is on the “one” side of the relationship, and the Orders table is on the “many” side. For you to implement this relationship, the field joining the two tables on the “one” side of the relationship must be unique.

In the Customers and Orders tables example, the CustomerID field that joins the two tables must be unique within the Customers table. If more than one customer in the Customers table has the same customer ID, it is not clear which customer belongs to an order in the Orders table. For this reason, the field that joins the two tables on the “one” side of the one-to-many relationship must be a primary key or have a unique index. In almost all cases, the field relating the two tables is the primary key of the table on the “one” side of the relationship. The field relating the two tables on the “many” side of the relationship is the foreign key.

One-to-One Relationships

In a one-to-one relationship, each record in the table on the “one” side of the relationship can have only one matching record in the table on the “many” side of the relationship. This relationship is not common and is used only in special circumstances. Usually, if you have set up a one-to-one relationship, you should have combined the fields from both tables into one table. The following are the most common reasons to create a one-to-one relationship:

  • The number of fields required for a table exceeds the number of fields allowed in an Access table.

  • Several fields in a table are required for only a subset of records in the table.

The maximum number of fields allowed in an Access table is 255. There are very few reasons a table should ever have more than 255 fields. In fact, before you even get close to 255 fields, you should take a close look at the design of the system. On the rare occasion when having more than 255 fields is appropriate, you can simulate a single table by moving some of the fields to a second table and creating a one-to-one relationship between the two tables.

The second situation in which you would want to define one-to-one relationships is when you will use certain fields in a table for only a relatively small subset of records. An example is an Employees table and a Vesting table. Certain fields are required only for employees who are vested. If only a small percentage of a company’s employees are vested, it is not efficient, in terms of performance or disk space, to place all the fields containing information about vesting in the Employees table. This is especially true if the vesting information requires a large number of fields. By breaking the information into two tables and creating a one-to-one relationship between the tables, you can reduce disk-space requirements and improve performance. This improvement is particularly pronounced if the Employees table is large.

Many-to-Many Relationships

In a many-to-many relationship, records in two tables have matching records. You cannot directly define a many-to-many relationship in Access; you must develop this type of relationship by adding a table called a junction table. You relate the junction table to each of the two tables in one-to-many relationships. For example, with an Orders table and a Products table, each order will probably contain multiple products, and each product is likely to be found on many different orders. The solution is to create a third table, called OrderDetails. You relate the OrderDetails table to the Orders table in a one-to-many relationship based on the OrderID field. You relate it to the Products table in a one-to-many relationship based on the ProductID field.

 
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