Relationships
The capability to define and manage relationships between data entities is the basis for much of the value of relational databases. Relationships allow data from one entity to be logically separated from others. With this separation, data can be modified without integrity loss and combined with other entities for analysis.
Data relationships in Force.com resemble those found in standard relational databases. You can express one-to-one, one-to-many, and many-to-many relationships. But relationships in Force.com are closely controlled and managed by the platform and also integrated with many platform features. Some important points are listed in the subsections that follow.
Integrity Enforced
When you define a relationship in Force.com, a relationship field is created to contain the foreign key. Force.com prevents you from using a foreign key to a different object. It enforces that the foreign key points to an object of the correct type.
This is basic foreign key constraint checking, like in a relational database. The difference in Force.com is that you can never elect to turn it off. It is a mandatory, always-on feature, protecting your data from inconsistency.
There is one minor exception to this rule. Many standard objects contain special fields that can be related to multiple object types. For example, a support case can be assigned to an individual user or a group representing a collection of users. In the Case object, the OwnerId field can contain the ID of a record in the User object or the Group object. Both types of foreign keys are valid. Note that polymorphic foreign key fields are defined by Salesforce and cannot be created by developers.
Explicitly Defined
In Force.com, all relationships are predefined, established when objects and fields are created. With the exception of semi- and anti-joins, you do not specify join conditions when you write queries. Instead, you specify the fields you want, and Force.com takes care of traversing the necessary relationships to retrieve the data.