Database Relationships Objective 5.01 Understand database tables used in business
- A relationship means that two or more tables are linked together by a common field, called a primary key field.
- Table relationships increase the power of the database by allowing data to be stored separately, but managed and retrieved collectively.
Making Friends with Relations Slide 5.01 Understand database tables used in business Relationships link data from individual tables and increase the usefulness of a database.
- One of the great benefits of working with databases is the ability to store huge quantities of information. One company’s database may contain multiple tables of related information.
- When the information between tables is linked, it is called a relationship.
What does a relationship look like? One record in the Student Table is related to one record in the Participation Table Relationship established between two tables
Keys to the Relationship
- A primary key is an essential element of a database table. It is a data type that is set to make each record within a database table unique and to link tables together.
The red line that graphically represents the relationship is called a Join Line
- Primary key fields must be of the same data type and size for the tables to which they link.
What makes a good primary key?
- Which of the following would make each record in a database unique?
Primary and Foreign Keys
- When tables relate, the primary key of one table becomes a foreign key of the other table
- For example, in the tables below,
- ISBN appears as a primary key in the Grocery Products table . . .…and in the Orders Table as a foreign key
Diagram of Primary & Foreign Keys 5.02 Understand queries, forms, and reports.
Function of the Junction Table
- A junction table is used to join primary key fields of multiple tables
In this example, Orders is the Junction Table. It allows the Grocery Products and the Customers tables to share information.
- The Junction table is like a hub or control center
- One record in Table A links to multiple records in Table B through the primary/foreign key fields of the two tables
- Multiple records in multiple tables are linked by their primary keys through a Junction Table.
- A many-to-many relationship is formed by connecting two or more tables that have a one-to-many relationship. The connection is made by a Junction Table.
Setting up a Relationship
The primary key in Grocery Products is ISBN The primary key in Orders is Order No The primary key in Customers is Store ID
- Add one table’s primary key to a field in another table that has the same properties
- Referential integrity protects related data that is stored in multiple tables.
- It would prevent a customer in one table from being deleted if the customer is also in a related table.
- A music store database contains three tables. An Inventory table is linked by artist to a Royalty table which contains the artist’s contact information and royalty percentages
- The Inventory table is also linked to a Sales table by salesperson ID numbers, which contains employee information and commission rates
What are the advantages of this system? Are there disadvantages?
- Once a purchase is made, the inventory is adjusted in one table and the artist’s royalties and employee’s commissions are calculated from the information in the other tables