general

[Former Member]

what is meant by foreign key relation ship?plz explain me clearly?

[Former Member]

Hi,

Foreign key

A foreign key creates a link between two tables T1 and T2. Every primary key field from T1 (check table) is assigned a field from table T2 (foreign key field). The fields from T2 assigned to primary key fields are marked as foreign key fields.

The most important function of the foreign key is the support of data integrity. The foreign key fields can only accept values which appear in the primary key of the check table. During input the values of the foreign key fields can thus be checked against the entries of the assigned key fields of the check table.

Foreign keys are also the foundation for defining lock objects, maintenance views and help views. These objects contain fields from several tables. All the tables used in such an object must be linked with foreign keys.

Jogdand M B

[Former Member]

Hi Naga,

A foreign key is an attribute that completes a relationship by identifying the parent entity. Foreign keys provide a method for maintaining integrity in the data (called referential integrity) and for navigating between different instances of an entity. Every relationship in the model must be supported by a foreign key.

You can define the relationships between tables in the ABAP Dictionary by creating foreign keys.Using foreign keys, you can easily create value checks for input fields. Foreign keys can also be used to link several tables in a view or a lock object.

A foreign key links two tables T1 and T2 by assigning fields of table T1 to the primary key fields of table T2.Table T1 is called the foreign key table (dependent table) and table T2 the check table (referenced table). The pair of fields for the two tables must have the same data type and length. One field of the foreign key table therefore corresponds to each key field of the check table. This field is called the foreign key field.

A foreign key permits you to assign data records in the foreign key table and check table. One record of the foreign key table uniquely identifies one record of the check table using the entries in the foreign key fields.

Hope this helps!

Regards,

Saurabh

[Former Member]

hi,

A foreign key is an attribute that completes a relationship by identifying the parent entity. Foreign keys provide a method for maintaining integrity in the data (called referential integrity) and for navigating between different instances of an entity. Every relationship in the model must be supported by a foreign key.

Identifying Foreign Keys

Every dependent and category (subtype) entity in the model must have a foreign key for each relationship in which it participates. Foreign keys are formed in dependent and subtype entities by migrating the entire primary key from the parent or generic entity. If the primary key is composite, it may not be split.

Foreign Key Ownership

Foreign key attributes are not considered to be owned by the entities to which they migrate, because they are reflections of attributes in the parent entities. Thus, each attribute in an entity is either owned by that entity or belongs to a foreign key in that entity.

If the primary key of a child entity contains all the attributes in a foreign key, the child entity is said to be "identifier dependent" on the parent entity, and the relationship is called an "identifying relationship." If any attributes in a foreign key do not belong to the child's primary key, the child is not identifier dependent on the parent, and the relationship is called "non identifying."

Diagramming Foreign Keys

Foreign keys attributes are indicated by the notation (FK) beside them. An example is shown in Figure 2 (b) above.

Primary and Foreign Keys

Primary and foreign keys are the most basic components on which relational theory is based. Primary keys enforce entity integrity by uniquely identifying entity instances. Foreign keys enforce referential integrity by completing an association between two entities. The next step in building the basic data model to

identify and define the primary key attributes for each entity

validate primary keys and relationships

migrate the primary keys to establish foreign keys

Define Primary Key Attributes

Attributes are data items that describe an entity. An attribute instance is a single value of an attribute for an instance of an entity. For example, Name and hire date are attributes of the entity EMPLOYEE. "Jane Hathaway" and "3 March 1989" are instances of the attributes name and hire date.

The primary key is an attribute or a set of attributes that uniquely identify a specific instance of an entity. Every entity in the data model must have a primary key whose values uniquely identify instances of the entity.

To qualify as a primary key for an entity, an attribute must have the following properties:

it must have a non-null value for each instance of the entity

the value must be unique for each instance of an entity

the values must not change or become null during the life of each entity instance

In some instances, an entity will have more than one attribute that can serve as a primary key. Any key or minimum set of keys that could be a primary key is called a candidate key. Once candidate keys are identified, choose one, and only one, primary key for each entity. Choose the identifier most commonly used by the user as long as it conforms to the properties listed above. Candidate keys which are not chosen as the primary key are known as alternate keys.

An example of an entity that could have several possible primary keys is Employee. Let's assume that for each employee in an organization there are three candidate keys: Employee ID, Social Security Number, and Name.

Name is the least desirable candidate. While it might work for a small department where it would be unlikely that two people would have exactly the same name, it would not work for a large organization that had hundreds or thousands of employees. Moreover, there is the possibility that an employee's name could change because of marriage. Employee ID would be a good candidate as long as each employee were assigned a unique identifier at the time of hire. Social Security would work best since every employee is required to have one before being hired.

Composite Keys

Sometimes it requires more than one attribute to uniquely identify an entity. A primary key that made up of more than one attribute is known as a composite key. Figure 1 shows an example of a composite key. Each instance of the entity Work can be uniquely identified only by a composite key composed of Employee ID and Project ID.

Regards,

Santosh