Data in one entity often relates to data in another entity. For example, you might have a Customers entity and an Orders entity, and the Orders entity might have a field to show which customer placed the order. You can use a lookup field to show data from the Customers entity based on which orders they placed.
You can create several types of relationships from one entity to another (or between an entity and itself). Each entity can have a relationship with more than one entity, and each entity can have more than one relationship to another entity. Some common relationship types are:
Normal - This type of relationship exists between two entities.
Self - This type of relationship exists between an entity and itself.
One-to-one - In this type of relationship, each record in entity A can match only one record in entity B, and vice versa.
One-to-many - In this type of relationship, each record in entity A can match more than one record in entity B, but each record in entity B can match only one record in entity A.
Many-to-many - In this type of relationship, each record in entity A can match more than one record in entity B, and vice versa. The current release of the Common Data Service doesn't support this type of relationship.
To add a lookup field to an entity, specify the field's data type as Lookup, and specify the entity with which you want to create a relationship.
If you create an app automatically from an entity that contains a lookup field, it appears as a Drop down control that contains data from the Title field of the referred entity.
If entity A has a lookup field to entity B:
Note: If entity B is a standard entity with a lookup relationship to entity A and you delete a record from entity B, all matched records in entity A are also deleted.
For information about how to delete a field, see Manage fields.