Creating multiple tables and table relationships
A relationship is established between two database tables when one table has a foreign key that references the primary key of another table. This is not a common relationship type, as the data stored in table B relationship between the Customers table and the Products table, we. Relationships. Relationships are a logical connection between different tables, established on the basis of interaction among.
The values that these fields contain should correspond with a value in the corresponding field in the referenced table. So any given value in Orders. CustomerId should also exist in the Customer.
Not good referential integrity.
Most database systems allow you to specify whether the database should enforce referential integrity. In our example, Orders. CustomerId field is a foreign key to the Customers. CustomerId which is the primary key of that table. ProductId field is a foreign key to the Products.
ProductId field which is the primary key of that table. It is crucial from a data-integrity standpoint because it helps to eliminate duplicate data and to keep redundant data to an absolute minimum. A typical example of a one-to-many relationship. Diagramming a one-to-many relationship. Note that the crow's foot symbol is always located next to the table on the "many" side of the relationship.
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Many-to-Many Relationships A pair of tables bears a many-to-many relationship when a single record in the first table can be related to one or more records in the second table and a single record in the second table can be related to one or more records in the first table. This is the second most common relationship that exists between a pair of tables in a database.
It can be a little more difficult to identify than a one-to-many relationship, so you must be sure to examine the tables carefully. A typical example of a many-to-many relationship. Diagramming a many-to-many relationship. In this case, there is a crow's foot symbol located next to each table. Problems with Many-to-Many Relationships A many-to-many relationship has an inherent peculiarity that you must address before you can effectively use the data from the tables involved in the relationship.
The issue is this: How do you easily associate records from the first table with records in the second table in order to establish the relationship?
This is an important question because you'll encounter problems such as these if you do not establish the relationship properly: It will be tedious and somewhat difficult to retrieve information from one of the tables. One of the tables will contain a large amount of redundant data. Duplicate data will exist within both tables. It will be difficult for you to insert, update, and delete data.
There are two common methods that novice and inexperienced developers use in a futile attempt to address this situation. Note As this example unfolds, keep in mind that every many-to-many relationship you encounter will exhibit these same issues. As you can see, there is no actual connection between the two tables, so you have no way of associating records in one table with records in the other table.
The first method you might use to attempt to establish a connection involves taking a field from one table and incorporating it a given number of times within the other table. This approach usually appeals to people who are accustomed to working with spreadsheets. Do these structures look vaguely familiar? All you've done using this method is introduce a "flattened" multivalued field into the table structure.
In doing so, you've also introduced the problems associated with a multivalued field. If necessary, review Chapter 7. Although you know how to resolve a multivalued field, this is not a good or proper way to establish the relationship.
Relational databases: Defining relationships between database tables
The second method you might attempt to use is simply a variation of the first method. In this case, you take one or more fields from one table and incorporate a single instance of each field within the other table. We already have our books and users tables, so we just need to create the cross-reference table: Each row of the checkouts table uses these two Foreign Keys to create an association between rows of users and books.
We can see on the first row of checkouts, the user with an id of 1 is associated with the book with an id of 1. On the second row, the same user is also associated with the book with an id of 2.
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On the third row a different user, with and id of 2, is associated with the same book from the previous row. On the fourth row, the user with an id of 5 is associated with the book with an id of 3. Don't worry if you don't completely understand this right away, we'll take a look shortly at what these associations look like in terms of the data in users and books. First, let's create our checkouts table and add some data to it.
While these aren't necessary to create the relationship between the users and books table, they can provide additional context to that relationship.
Attributes like a checkout date or return date don't pertain specifically to users or specifically to books, but to the association between a user and a book. This kind of additional context can be useful within the business logic of the application using our database. Now that we have our checkouts created, we can add the data that will create the associations between the rows in users and books.Entity Relationship Diagram (ERD) Tutorial - Part 1
We can perhaps think of a Many-to-Many relationship as combining two One-to-Many relationships; in this case between checkouts and users, and between checkouts and books. Summary In this chapter we covered a number of different topics regarding table relationships: We briefly covered normalization, and how this is used to reduce redundancy and improve data integrity within a database.
ERDs were introduced, and we discussed how these diagrams allow us to model the relationships between different entities. We also looked at keys, and how Primary and Foreign keys, and how these work together to create the relationships between different tables.