database - One-to many relationships in ER diagram - Stack Overflow
Check out this guide for a comprehensive look at all ER diagram symbols Conceptual ERDs can be used as the foundation for logical data models. Attributes are characteristics of an entity, a many-to-many relationship, or a one- to-one. CS Home Data Modeling ERD Entities Relationships Attributes Example A single entity instance in one entity class (parent) is related to multiple entity. Our sales database represents one of these patterns. There are two additional attributes shown in the class diagram that we haven't talked about yet. Products has turned into a one-to-many relationship between Orders and Order Lines.
One-to-many (data model)
In this case I chose EmployeeTask over Assigned. Notice the multiplicities in Figure The rule is that the multiplicities "cross over" once the associative table is introduced, as indicated in Figure A multiplicity of 1 is always introduced on the outside edges of the relationship within the data schema to preserve overall multiplicity of the original relationship.
The original relationship indicated that an employee is assigned to one or more tasks and that a task has zero or more employees assigned to it. In the data schema you see that this is still true even with the associative table in place to maintain the relationship.
Assume that an employee object is in memory and we need a list of all the tasks they have been assigned. The steps that the application would need to go through are: The Select statement is run against the database. The data records representing these tasks are marshaled into Task objects.
Part of this effort includes checking to see if the Task object is already in memory. If it is then we may choose to refresh the object with the new data values this is a concurrency issue. A similar process would have been followed to read in the employees involved in a given task.
To save the relationship, still from the point of view of the Employee object, the steps would be: Add Update statements for any task objects that have changed. Add Insert statements for the Task table for any new tasks that you have created. Add Insert statements for the EmployeeTask table for the new tasks. Add Delete statements for the Task table any tasks that have been deleted. This may not be necessary if the individual object deletions have already occurred.
Add Delete statements for the EmployeeTask table for any tasks that have been deleted, a step that may not be needed if the individual deletions have already occurred.
Add Delete statements for the EmployeeTask table for any tasks that are no longer assigned to the employee. Many-to-many relationships are interesting because of the addition of the associative table.
7. One-to-many (or many-to-one) relationships
Two business classes are being mapped to three data tables to support this relationship, so there is extra work to do as a result. When mapping this to a relational database you need to add an addition column to track this information.
The database schema, also depicted in Figure 1includes the column OrderItem. ItemSequence to persist this information. Although this mapping seems straightforward on the surface, there are several issues that you need take into consideration. These issues become apparent when you consider basic persistence functionality for the aggregate: Read the data in the proper sequence. The scaffolding attribute that implements this relationship must be a collection that enables sequential ordering of references and it must be able to grow as new OrderItems are added to the Order.
In Figure 2 you see that a Vector is used, a Java collection class that meets these requirements. As you read the order and order items into memory the Vector must be filled in the proper sequence. If the values of the OrderItem. ItemSequence column start from 1 and increase by 1 then you can simply use the value of the column as the position to insert order items into the collection.
You have an order with five order items in memory and they have been saved into the database. You now insert a new order item in between the second and third order items, giving you a total of six order items.
With the current data schema of Figure 1 you have to renumber the sequence numbers for every order item that appears after the new order item and then write out all them even though nothing has changed other than the sequence number in the other order items.
Database Design - Many-to-many
Because the sequence number is part of the primary key of the OrderItem table this could be problematic if other tables, not shown in Figure 1refer to rows in OrderItem via foreign keys that include ItemSequence.
When do you update sequence numbers after rearranging the order items? Whenever you rearrange order items on an order, perhaps you moved the fourth order item to be the second one on the order, you need to update the sequence numbers within the database.
- Design pattern: many-to-many (order entry)
Do you update sequence numbers after deleting an order item? If you delete the fifth of six order items do you want to update the sequence number for what is now the fifth item or do you want to leave it as it. The sequence numbers still work — the values are 1, 2, 3, 4, 6 — but you can no longer use them as the position indicators within your collection without leaving a hole in the fifth position.Access 2007 Tutorial 1.3. Many to Many Relationship
Consider sequence number gaps greater than one. Each would be a different instance of this class. Each time an order is placed for a product, we need to know how many units of that product are being ordered and what price we are actually selling the product for. The sale price might vary from the list price by customer discount, special sale, etc.
One-to-many (data model) - Wikipedia
These attributes are a result of the association between the Order and the Product. We show them in an association class that is connected to the association by a dotted line. If there are no attributes that result from a many-to-many association, there is no association class.
Class diagram Other views of this diagram: We need to know the subtotal for each order line that is, the quantity times the unit sale price and the total dollar value of each order the sum of the subtotals for each line in that order. So for every many-to-many, we will need a junction table in the database, and we need to show the scheme of this table in our diagram.
If there is an association class like OrderLinesits attributes will go into the junction table scheme.
If there is no association class, the junction table sometimes also called a join table or linking table will contain only the FK attributes from each side of the association. Other views of this diagram: Large image - Data dictionary text The many-to-many association between Orders and Products has turned into a one-to-many relationship between Orders and Order Lines, plus a many-to-one relationship between Order Lines and Products. You should also describe these in English, to be sure that you have the fk's in the right place: UPCs are defined for virtually all grocery and manufactured products by a commercial organization called the Uniform Code Council, Inc.
To uniquely identify each order line, we need to know both which order this line is contained in, and which product is being ordered on this line. The two fk's, from Orders and Products, together form the only candidate key of this relation and therefore the primary key. There is no need to look for a smaller pk, since OrderLines has no children.