More Database Design Errors – Confusion with Many-to-Many Relationships
A many-to-many relationship depends for its existence on the underlying entities. Relational databases and SQL have robust support for. A relational database organizes data in tables (or relations). A table is made Database design is more art than science, as you have to make many decisions. This article introduces a design pattern for the many-to-many relationship, and shows the UML class diagram which is then mapped to the relational model.
We refer to these as design patterns. Many-to-many relationships Introduction As we learned in the article on associationsobjects interact with each other and we use relationships in our model of these objects to capture these interactions. In that section we learned about 1-to-many relationships. There are also many-to-many and one-to-one relationships; as you can see these three types of relationships are named after the cardinality constraints.
There is a fourth type, the many-to-one relationship, however, that is just the reverse direction of the one-to-many relationship. This article introduces a design pattern for the many-to-many relationship, and shows the UML class diagram which is then mapped to the relational model. The sales database So far our sales database has customers and orders. To finish the design pattern, we need products to sell.
Below is how Product might be described A product is a specific type of item that we have for sale. For each product, we need to know its unit list price and how many units of this product we have in stock.
It is important to understand exactly what is meant by a product so that it can be modeled correctly. This is a type of product, not an individual boxed blender that is sitting on our shelves.
The same manufacturer probably has different blender models,and there are probably blenders that we stock that are made by other companies. Each such blender would be a different instance of this class. The following would be a typical description of how products relate with orders. An order can include many products, though a product will only be in an order once. Each time an order is placed for a product, we need to know how many units of that product are being ordered and the price we are actually selling the product for.
The latter is needed as the sale price might vary from the list price by customer discount, special sale, price changes, etc. From the above description, we can extract the following sentences which focus on the constraints that will need to be modeled. Each Order must contain one and at most many Products.
The rest tells us that an order can contain many products.
Each Product may be contained in many Orders. Of course, we need to allow a product to be in many orders. There are significant differences in this modeling problem compared to what we have seen before; these are listed below. The maximum multiplicity i. We will see how to model this in UML and then how to map this to the relational model.
There are two attributes described, the sale price and the quantity of the product being ordered, that cannot possibly be attributes of either the Products or Orders classes. These attributes are a result of the association between the Order and the Product, they describe each individual instance of the association.
Below, we learn how to model such attributes in UML and then how to implement these in the relational model. Class diagram The many-to-many relationship will be modeled with an association that specifies a many cardinality in the multiplicity constraints of both ends of the association. UML gives us a way to model relationship attributes by showing them in an association class that is connected to the association by a dotted line.
This is shown in the figure below with the OrderLine association class and the two attributes quantity and unitSalePrice as described above. If no information needs to be modeled about specific associations between objects such as the quantity and unitSalePricethen the UML class diagram omits the association class.
Take note that for every value in the parent table, there could be zero, one, or more rows in the child table. For every value in the child table, there is one and only one row in the parent table. Many-to-Many In a "product sales" database, a customer's order may contain one or more products; and a product can appear in many orders. In a "bookstore" database, a book is written by one or more authors; while an author may write zero or more books.
This kind of relationship is known as many-to-many. Let's illustrate with a "product sales" database.
Relational Database Design
We begin with two tables: The table products contains information about the products such as name, description and quantityInStock with productID as its primary key.
The table orders contains customer's orders customerID, dateOrdered, dateRequired and status. Again, we cannot store the items ordered inside the Orders table, as we do not know how many columns to reserve for the items. We also cannot store the order information in the Products table. To support many-to-many relationship, we need to create a third table known as a junction tablesay OrderDetails or OrderLineswhere each row represents an item of a particular order.
For the OrderDetails table, the primary key consists of two columns: The many-to-many relationship is, in fact, implemented as two one-to-many relationships, with the introduction of the junction table.
How to Handle a Many-to-Many Relationship in Database Design
An order has many items in OrderDetails. An OrderDetails item belongs to one particular order.
A product may appears in many OrderDetails. Each OrderDetails item specified one product. One-to-One In a "product sales" database, a product may have optional supplementary information such as image, moreDescription and comment.
- Design pattern: many-to-many (order entry)
- A Quick-Start Tutorial on Relational Database Design
Keeping them inside the Products table results in many empty spaces in those records without these optional data. Furthermore, these large data may degrade the performance of the database.
A record will only be created for those products with optional data. The two tables, Products and ProductDetails, exhibit a one-to-one relationship. That is, for every row in the parent table, there is at most one row possibly zero in the child table.
The same column productID should be used as the primary key for both tables. Some databases limit the number of columns that can be created inside a table. You could use a one-to-one relationship to split the data into two tables. One-to-one relationship is also useful for storing certain sensitive data in a secure table, while the non-sensitive ones in the main table.
Column Data Types You need to choose an appropriate data type for each column. Commonly data types include: Normalization Apply the so-called normalization rules to check whether your database is structurally correct and optimal.
First Normal Form 1NF: A table is 1NF if every cell contains a single value, not a list of values. This properties is known as atomic.
The 3 Types of Relationships in Database Design | posavski-obzor.info
Instead, you should create another table using one-to-many relationship. Second Normal Form 2NF: A table is 2NF, if it is 1NF and every non-key column is fully dependent on the primary key. Furthermore, if the primary key is made up of several columns, every non-key column shall depend on the entire set and not part of it.
If unitPrice is dependent only on productID, it shall not be kept in the OrderDetails table but in the Products table. On the other hand, if the unitPrice is dependent on the product as well as the particular order, then it shall be kept in the OrderDetails table.
Third Normal Form 3NF: A table is 3NF, if it is 2NF and the non-key columns are independent of each others. In other words, the non-key columns are dependent on primary key, only on the primary key and nothing else.
For example, suppose that we have a Products table with columns productID primary keyname and unitPrice. The column discountRate shall not belong to Products table if it is also dependent on the unitPrice, which is not part of the primary key. At times, you may decide to break some of the normalization rules, for performance reason e.
Make sure that you fully aware of it, develop programming logic to handle it, and properly document the decision.
Integrity Rules You should also apply the integrity rules to check the integrity of your design: The primary key cannot contain NULL. Otherwise, it cannot uniquely identify the row. For composite key made up of several columns, none of the column can contain NULL. Each foreign key value must be matched to a primary key value in the table referenced or parent table.
You can insert a row with a foreign key in the child table only if the value exists in the parent table. If the value of the key changes in the parent table e. You could either a disallow the changes; b cascade the change or delete the records in the child tables accordingly; c set the key value in the child tables to NULL.
Most RDBMS can be setup to perform the check and ensure the referential integrity, in the specified manner. Beside the above two general integrity rules, there could be integrity validation pertaining to the business logic, e. These could be carried out in validation rule for the specific column or programming logic.
Column Indexing You could create index on selected column s to facilitate data searching and retrieval. A specialized index e. However, the index needs to be rebuilt whenever a record is changed, which results in overhead associated with using indexes.Database Design Tutorial
Index can be defined on a single column, a set of columns called concatenated indexor part of a column e. You could built more than one index in a table. For example, if you often search for a customer using either customerName or phoneNumber, you could speed up the search by building an index on column customerName, as well as phoneNumber.