Bookshelf: Primary Key-Foreign Key Relationships
This Oracle tutorial explains how to use Foreign Keys in Oracle with syntax and The foreign key in the child table will generally reference a primary key in the. One to many using a primary-key and foreign-key relationship: Add Foreign Key «Constraints «Oracle PL / SQL. Oracle Database Tips by Donald BurlesonDecember 16, A foreign key constraint is used to enforce a relationship between two tables. This is because it is a composite primary key and composite primary keys have to be defined out .
You can use sequence numbers to easily generate numeric primary keys. The purpose of these contraints is different from that of primary keys. Unique key constraints are appropriate for any column where duplicate values are not allowed.
Primary keys identify each row of the table uniquely, and typically contain values that have no significance other than being unique. Some examples of good unique keys include: An employee's social security number the primary key is the employee number A truck's license plate number the primary key is the truck number A customer's phone number, consisting of the two columns AREA and PHONE the primary key is the customer number A department's name and location the primary key is the department number Constraints On Views for Performance, Not Data Integrity The constraints discussed throughout this chapter apply to tables, not views.
Although you can declare constraints on views, such constraints do not help maintain data integrity. Instead, they are used to enable query rewrites on queries involving views, which helps performance with materialized views and other data warehousing features. The constraints are never enforced, and there is no associated index. Oracle9i Data Warehousing Guide for information on query rewrite, materialized views, and the performance reasons for declaring constraints on views. Enforcing Referential Integrity with Constraints Whenever two tables contain one or more common columns, Oracle can enforce the relationship between the two tables through a referential integrity constraint.
Depending on this relationship, you may want to define additional integrity constraints including the foreign key, as listed in the section "Defining Relationships Between Parent and Child Tables".
It guarantees that every value in this column must match a value in the primary key of the department table. This constraint prevents erroneous department numbers from getting into the employee table. Foreign keys can be comprised of multiple columns. Such a composite foreign key must reference a composite primary or unique key of the exact same structure, with the same number of columns and the same datatypes.
Because composite primary and unique keys are limited to 32 columns, a composite foreign key is also limited to 32 columns. To enforce the "match full" rule for nulls in composite foreign keys, which requires that all components of the key be null or all be non-null, define a CHECK constraint that allows only all nulls or all non-nulls in the composite foreign key. For example, with a composite key comprised of columns A, B, and C: Figure Tables with Referential Integrity Constraints Text description of the illustration adg No Constraints on the Foreign Key When no other constraints are defined on the foreign key, any number of rows in the child table can reference the same parent key value.Oracle - SQL - Foreign Key Constraint
This model allows nulls in the foreign key. This model establishes a "one-to-many" relationship between the parent and foreign keys that allows undetermined values nulls in the foreign key. An example of such a relationship is shown in Figure on page 8 between the employee and department tables. Each department parent key has many employees foreign keyand some employees might not be in a department nulls in the foreign key. NOT NULL Constraint on the Foreign Key When nulls are not allowed in a foreign key, each row in the child table must explicitly reference a value in the parent key because nulls are not allowed in the foreign key.
However, any number of rows in the child table can reference the same parent key value. This model establishes a "one-to-many" relationship between the parent and foreign keys. However, each row in the child table must have a reference to a parent key value; the absence of a value a null in the foreign key is not allowed.
The same example in the previous section can be used to illustrate such a relationship. However, in this case, employees must have a reference to a specific department.
Oracle Foreign Key Constraint: Enforcing Relationship Beween Tables
This model establishes a "one-to-one" relationship between the parent and foreign keys that allows undetermined values nulls in the foreign key. Because nulls are not allowed in the foreign key, each row in the child table must explicitly reference a value in the parent key. This model establishes a "one-to-one" relationship between the parent and foreign keys that does not allow undetermined values nulls in the foreign key.
This situation might be present if a single column is part of two different composite foreign keys. Deferring Constraint Checks When Oracle checks a constraint, it signals an error if the constraint is not satisfied. Tables are snapshots Tables that contain a large amount of data being manipulated by another application, which may or may not return the data in the same order Update cascade operations on foreign keys When dealing with bulk data being manipulated by outside applications, you can defer checking constraints for validity until the end of a transaction.
You can do so by issuing a statement similar to the following: Use the following DML statement to set all deferrable constraints deferred: The defaults specified when you create a constraint remain as long as the constraint exists. If there are any problems with a constraint, this statement will fail and the constraint causing the error will be identified. If you commit while constraints are violated, the transaction will be rolled back and you will receive an error message.
If there is no such index, Oracle creates one. Minimizing Space and Time Overhead for Indexes Associated with Constraints When Oracle uses a unique index to enforce a constraint, and constraints associated with the unique index are dropped or disabled, the index is dropped. The only exception is when the matching unique or primary key is never updated or deleted.
Oracle9i Database Concepts for information on locking mechanisms involving indexes and keys. About Referential Integrity in a Distributed Database The declaration of a referential integrity constraint cannot specify a foreign key that references a primary or unique key of a remote table. If you decide to define referential integrity across the nodes of a distributed database using triggers, be aware that network failures can make both the parent table and the child table inaccessible.
If the network connection between the two databases fails, then some DML statements against the child table those that insert rows or update a foreign key value cannot proceed, because the referential integrity triggers must have access to the parent table in the HQ database. Never use CHECK constraints when any of the other types of integrity constraints can provide the necessary checking. A CHECK constraint on the salary and commissions columns to prevent the commission from being larger than the salary.
If a statement causes the condition to evaluate to false, then the statement is rolled back. The condition must be a boolean expression that can be evaluated using the values in the row being inserted or updated.
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The condition cannot contain subqueries or sequences. Oracle9i SQL Reference for an explanation of these pseudocolumns.
The condition cannot contain a user-defined SQL function. The order in which the constraints are evaluated is not defined, so be careful not to rely on the order or to define multiple constraints that conflict with each other. In the case where a composite key can allow only all nulls or all values, you must use a CHECK integrity constraint.
For example, the following expression of a CHECK integrity constraint allows a key value in the composite key made up of columns C1 and C2 to contain either all nulls or all values: Notice how all constraints are given a name. Naming the constraints prevents the database from creating multiple copies of the same constraint, with different system-generated names, if the DDL is run multiple times.
Oracle9i Database Administrator's Guide for information on creating and maintaining constraints for a large production database. This name must be unique with respect to other constraints that you own. If you do not specify a constraint name, one is assigned by Oracle. Picking your own name makes error messages for constraint violations more understandable, and prevents the creation of multiple constraints if the SQL statements are run more than once.
Note that the name of each constraint is included with other information about the constraint in the data dictionary. Enabling and Disabling Integrity Constraints This section explains the mechanisms and procedures for manually enabling and disabling integrity constraints.
When a constraint is enabled, the corresponding rule is enforced on the data values in the associated columns. The definition of the constraint is stored in the data dictionary. When a constraint is disabled, the corresponding rule is not enforced. The definition of the constraint is still stored in the data dictionary. An integrity constraint represents an assertion about the data in a database. This assertion is always true when the constraint is enabled.
The assertion may or may not be true when the constraint is disabled, because data that violates the integrity constraint can be in the database.
During day-to-day operations, constraints should always be enabled.
How to Create a Foreign Key on a Table in Oracle SQL
In certain situations, temporarily disabling the integrity constraints of a table makes sense for performance reasons. About Exceptions to Integrity Constraints If a row of a table disobeys an integrity constraint, then this row is in violation of the constraint and is called an exception to the constraint. If any exceptions exist, then the constraint cannot be enabled. The rows that violate the constraint must be either updated or deleted before the constraint can be enabled.
Delete or update the row from the parent table, and set the foreign key column or columns in the child table to NULL. Rejects the delete or update operation for the parent table. A keyword from standard SQL. The MySQL Server rejects the delete or update operation for the parent table if there is a related foreign key value in the referenced table.
MySQL supports foreign key references between one column and another within a table. A column cannot have a foreign key reference to itself. Examples of Foreign Key Clauses Here is a simple example that relates parent and child tables through a single-column foreign key: One foreign key references a two-column index in the product table.
The other references a single-column index in the customer table: The syntax relating to foreign keys for this statement is shown here: The server prohibits changes to foreign key columns with the potential to cause loss of referential integrity. This avoids problems with tables having to be reloaded in a particular order when the dump is reloaded. It is also possible to set this variable manually: