Question:
I need to understand how to add a foreign key constraint statement to
make sure that all rows exists in the master table. How do you alter a
column value to make it into a foreign key constraint?
Foreign Key Constraints
A foreign key constraint is used to enforce a
relationship between two tables. As an example, take the case of two tables,
ITEM and PART. These tables have a relationship (an item can have none, one or
many parts). Foreign key constraints help to enforce that relationship. In the
DBA world, we use diagrams called Entity Relationship Diagrams (ERD) to show
visually how tables relate. Here is an ERD that shows how the ITEM and PART
tables relate.
Notice we have the ITEM and PART tables diagramed in two
boxes. There is a line between the two. Now, notice the two lines from the PART
table to the main line between the ITEM and PART tables. We call these ?crows
feet?. When you see crows feet, this means that there is a one to many
relationship between the tables.
In the diagram above, there is a one-to-many
relationship between the ITEM and PART tables. An item may have no parts
assigned to build it (perhaps it's new and in development), or it may have one
or many parts that go into building the part. Notice the circle at the top of
the crow's feet. Since it is not filled in, that indicates that there may not be
any parts in the part table. If the circle was darkened, this would indicate
that there must be at least one part associated with each item.
So, how does this relate to foreign key constraints?
Well, foreign key constraints help to enforce the types of relationships between
tables we have just demonstrated. In this example we will create the ITEM and
PART table. In the process of doing so, we will create a foreign key
relationship between the two:
CREATE
TABLE part(
Part_no NUMBER PRIMARY KEY,
Part_desc
VARCHAR2(200) NOT NULL );
CREATE
TABLE item (
Item_no
NUMBER,
Part_no
NUMBER,
Item_desc
varchar2(200) NOT NULL,
CONSTRAINT fk_item_part FOREIGN KEY (part_no) REFERENCES PART (part_no),
CONSTRAINT pk_item PRIMARY KEY (item_no, part_no) );
In this example, what we are really interested in is the
creation of the ITEM table. First, note that we defined the primary key as an
out of line primary key. This is because it is a composite primary key and
composite primary keys have to be defined out of line.
Now, we are interested in the foreign key definition.
You must define foreign key constraints as out of line constraints, as we have
done in our example. Here is a snippet of the command that we used:
CONSTRAINT fk_item_part FOREIGN KEY (part_no) REFERENCES
part (part_no);
Note that we start out using the constraint keyword.
This tells Oracle that we are about to define a constraint. Then, we name the
constraint. In this case we named the constraint FK_ITEM_PART. Constraint names
have to be unique for each table and can be no more than 30 characters in
length. We then use the foreign key keyword to indicate that we are defining a
foreign key constraint. Next we define the column in the table being created
that this foreign key belongs to.
Next we use the references keyword to indicate that we
are going to define the table and column that this foreign key references. The
referenced table name is next listed. In this case, it's the PART table. Finally
the column in the PART table that is being referenced is listed, PART_NO.
The bottom line is that now, a part cannot be added to
the ITEM table, unless it's listed in the PART table. This ensures data
integrity is maintained. If you need to add foreign key constraints to a table
after the fact, simply use the alter table command as seen here:
ALTER TABLE my_status ADD CONSTRAINT fk_my_status
FOREIGN KEY (part_no) REFERENCES part (part_no);
Foreign key
constraint example to enforce a business rule
The referential integrity, foreign key constraint, identifies
a column of a table or view as the foreign key. This foreign
key column establishes a direct relationship with a primary
key or unique key column (referenced key) usually in another
table. The table containing the foreign key is referred to as
the child, and the table containing the referenced key is the
parent table. The foreign key and referenced key can be in the
same table (parent and child are the same table); otherwise,
the parent and child tables must be in the same database.
Here's the syntax for creating a foreign key constraint:
ALTER TABLE (table_name)
ADD
CONSTRAINT (foreign key constraint name)
FOREIGN KEY
( field name )
REFERENCES primary_table_name
(primary_table_primary_index_field)
Using our example listed above in the references constraint,
the coding would appear as follows:
ALTER TABLE
dependent
ADD
CONSTRAINT ( emp_num_fk )
FOREIGN KEY
( emp_num )
REFERENCES employee ( emp_num );
While foreign key constraints should always be used in lieu of
application-only coded integrity checking, it's important to
realize that using referential integrity constraints adds I/O
overhead to insert and update operations. The
additional I/O is attributable to the fact that the foreign
key must be validated with the parent table. This overhead can
be minimized by assuring that referenced columns are indexed.
During bulk loads, it can be advantageous to disable integrity
constraints prior to the load and re-enable the constraints
after the load is completed. Once constraints are re-enabled,
any errors can be noted and handled.
Referential integrity sometimes needs to be double coded: once
for the database and again within the application. For
example, in a multipart SQL*Form, you may not become aware of
a referential integrity violation until you are many pages
into the form and your form attempts to commit the
transaction. In Web Server applications, you do not have the
luxury of making a lot of queries against the database, and
you need to be careful to keep the Oracle transactions as few
as possible.
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