In What Is Normalization, Part III, I discussed how to create a relationship to join our normalized tables back together so that no information was lost in the normalization process.
There are three basic kinds of relationships: One-to-One, One-to-Many, and Many-to-Many.
A marriage is an example of a 1:1 relationship. A husband can have one and only one wife and a wife can have one and only one husband. Of course, in the real world, there are exceptions to this, but let's accept this as a general case.
To implement a one-to-one relationship, you must have a primary key in both databases, which stores the same information, SS# for example. So continuing with our employee database example, suppose we had a ContactInfo table which stored Address, City, State, Zip, and Phone.
In this simplified example, we'll assume that people have a single address and phone. The primary key for this table would also be SS# since each record in the ContactInfo table represents the contact information for a single employee.
To create the one-to-one relationship, I would click and drag the primary key from Employee (SS#) to the primary key of ContactInfo (also SS#). Although the fields are named the same in this example, they do not have to be, however, they MUST represent the same information. In the Relationship Window, it would look like Figure 1.
Figure 1: One-to-One Relationship
Notice there is a 1 on each end of the relationship.
In relational databases, 1:1 relationships have limited utility. In most cases, the data elements from both tables can be combined into a single table.
This type of relationship is also known as the parent/child relationship for good reason. If we consider a biological parents only, we can say each mother can have one or more children and each child can have one and only one mother.
As I showed in Part III, the relationship between Employee and Salary History is a one-to-many relationship where each record in the employee table can be related to multiple records in the Salary History table, but each record in Salary History is related to only one record in Employee.
To implement this type of relationship, you must have a field in the "child" table that corresponds to the primary key in the "parent" table. Again, this field is SS# in our example, However, unlike the one-to-one relationship, it is NOT the primary key of Salary History, although it may be part of the primary key. Figure 2 shows this relationship.
Figure 2: One-to-Many Relationship
One-to-Many relationships are extremely common in relational databases.
In real life, M:M relationships are extremely common. You'll notice that in both the other illustrations, I had to qualify the definition of both a marriage and parent/child. That's because real world definitions are messy. In some places in the world, a man can have multiple wives and a woman, multiple husbands. Sometimes simultaneously. Children can have more than one parent. In the real world, these relationships many-to-many.
Because many-to-many relationships are common in the real world, we must also know how to represent them in relational databases. Unfortunately, M:M relationships cannot be represented directly. They must be represented indirectly by a linking table and a pair of 1:M relationships.
In What Is Normalization: Part V, I'll show how.