Now
that you have divided your information into tables, you need a way to
bring the information together again in meaningful ways.
Microsoft
Access, SQL server, MySql and Oracle relational database management
system. In a relational database, you divide your information into
separate, subject-based tables. You then use table relationships to
bring the information together as needed.
Creating a one-to-many relationship
Consider this example: the Suppliers and Products tables in the product orders database. A supplier can supply any number of products. It follows that for any supplier represented in the Suppliers table, there can be many products represented in the Products table. The relationship between the Suppliers table and the Products table is, therefore, a one-to-many relationship.
Creating a many-to-many relationship
Consider the relationship between the Products table and Orders table.
A single order can include more than one product. On the other hand, a single product can appear on many orders. Therefore, for each record in the Orders table, there can be many records in the Products table. And for each record in the Products table, there can be many records in the Orders table. This type of relationship is called a many-to-many relationship because for any product, there can be many orders; and for any order, there can be many products. Note that to detect many-to-many relationships between your tables, it is important that you consider both sides of the relationship.
The subjects of the two tables — orders and products — have a many-to-many relationship. This presents a problem. To understand the problem, imagine what would happen if you tried to create the relationship between the two tables by adding the Product ID field to the Orders table. To have more than one product per order, you need more than one record in the Orders table per order. You would be repeating order information for each row that relates to a single order — resulting in an inefficient design that could lead to inaccurate data. You run into the same problem if you put the Order ID field in the Products table — you would have more than one record in the Products table for each product. How do you solve this problem?
The answer is to create a third table, often called a junction table or link table, that breaks down the many-to-many relationship into two one-to-many relationships. You insert the primary key from each of the two tables into the third table. As a result, the third table records each occurrence or instance of the relationship.
Consider the relationship between the Products table and Orders table.
A single order can include more than one product. On the other hand, a single product can appear on many orders. Therefore, for each record in the Orders table, there can be many records in the Products table. And for each record in the Products table, there can be many records in the Orders table. This type of relationship is called a many-to-many relationship because for any product, there can be many orders; and for any order, there can be many products. Note that to detect many-to-many relationships between your tables, it is important that you consider both sides of the relationship.
The subjects of the two tables — orders and products — have a many-to-many relationship. This presents a problem. To understand the problem, imagine what would happen if you tried to create the relationship between the two tables by adding the Product ID field to the Orders table. To have more than one product per order, you need more than one record in the Orders table per order. You would be repeating order information for each row that relates to a single order — resulting in an inefficient design that could lead to inaccurate data. You run into the same problem if you put the Order ID field in the Products table — you would have more than one record in the Products table for each product. How do you solve this problem?
The answer is to create a third table, often called a junction table or link table, that breaks down the many-to-many relationship into two one-to-many relationships. You insert the primary key from each of the two tables into the third table. As a result, the third table records each occurrence or instance of the relationship.
Each record in the Order Details table represents one line item on an order. The Order Details table’s primary key consists of two fields — the foreign keys from the Orders and the Products tables. Using the Order ID field alone doesn’t work as the primary key for this table, because one order can have many line items. The Order ID is repeated for each line item on an order, so the field doesn’t contain unique values. Using the Product ID field alone doesn’t work either, because one product can appear on many different orders. But together, the two fields always produce a unique value for each record.
In the product sales database, the Orders table and the Products table are not related to each other directly. Instead, they are related indirectly through the Order Details table. The many-to-many relationship between orders and products is represented in the database by using two one-to-many relationships:
The Orders table and Order Details table have a one-to-many relationship. Each order can have more than one line item, but each line item is connected to only one order.
The Products table and Order Details table have a one-to-many relationship. Each product can have many line items associated with it, but each line item refers to only one product.
From the Order Details table, you can determine all of the products on a particular order. You can also determine all of the orders for a particular product.
Creating a one-to-one relationship
Another type of relationship is the one-to-one relationship. For instance, suppose you need to record some special supplementary product information that you will need rarely or that only applies to a few products. Because you don't need the information often, and because storing the information in the Products table would result in empty space for every product to which it doesn’t apply, you place it in a separate table. Like the Products table, you use the ProductID as the primary key. The relationship between this supplemental table and the Product table is a one-to-one relationship. For each record in the Product table, there exists a single matching record in the supplemental table. When you do identify such a relationship, both tables must share a common field.
When you detect the need for a one-to-one relationship in your database, consider whether you can put the information from the two tables together in one table. If you don’t want to do that for some reason, perhaps because it would result in a lot of empty space, the following list shows how you would represent the relationship in your design:
If the two tables have the same subject, you can probably set up the
relationship by using the same primary key in both tables.
If the two tables have different subjects with different primary keys, choose one of the tables (either one) and insert its primary key in the other table as a foreign key.
Determining the relationships between tables helps you ensure that you have the right tables and columns. When a one-to-one or one-to-many relationship exists, the tables involved need to share a common column or columns. When a many-to-many relationship exists, a third table is needed to represent the relationship.
If the two tables have different subjects with different primary keys, choose one of the tables (either one) and insert its primary key in the other table as a foreign key.
Determining the relationships between tables helps you ensure that you have the right tables and columns. When a one-to-one or one-to-many relationship exists, the tables involved need to share a common column or columns. When a many-to-many relationship exists, a third table is needed to represent the relationship.
