DB2 Version 10.1 for Linux, UNIX, and Windows

Foreign key (referential) constraints

Foreign key constraints (also known as referential constraints or referential integrity constraints) enable you to define required relationships between and within tables.

For example, a typical foreign key constraint might state that every employee in the EMPLOYEE table must be a member of an existing department, as defined in the DEPARTMENT table.

Referential integrity is the state of a database in which all values of all foreign keys are valid. A foreign key is a column or a set of columns in a table whose values are required to match at least one primary key or unique key value of a row in its parent table. A referential constraint is the rule that the values of the foreign key are valid only if one of the following conditions is true:

They appear as values of a parent key.
Some component of the foreign key is null.

To establish this relationship, you would define the department number in the EMPLOYEE table as the foreign key, and the department number in the DEPARTMENT table as the primary key.

Figure 1 shows how a record with an invalid key is prevented from being added to a table when a foreign key constraint exists between two tables:

Figure 1. Foreign and primary key constraints

Employee with invalid department number cannot be added to the employee table.

The table containing the parent key is called the parent table of the referential constraint, and the table containing the foreign key is said to be a dependent of that table.

Referential constraints can be defined in the CREATE TABLE statement or the ALTER TABLE statement. Referential constraints are enforced by the database manager during the execution of INSERT, UPDATE, DELETE, ALTER TABLE, MERGE, ADD CONSTRAINT, and SET INTEGRITY statements.

Referential integrity rules involve the following terms:

Table 1. Referential integrity terms
Concept	Terms
Parent key	A primary key or a unique key of a referential constraint.
Parent row	A row that has at least one dependent row.
Parent table	A table that contains the parent key of a referential constraint. A table can be a parent in an arbitrary number of referential constraints. A table that is the parent in a referential constraint can also be the dependent in a referential constraint.
Dependent table	A table that contains at least one referential constraint in its definition. A table can be a dependent in an arbitrary number of referential constraints. A table that is the dependent in a referential constraint can also be the parent in a referential constraint.
Descendent table	A table is a descendent of table T if it is a dependent of T or a descendent of a dependent of T.
Dependent row	A row that has at least one parent row.
Descendent row	A row is a descendent of row r if it is a dependent of r or a descendent of a dependent of r.
Referential cycle	A set of referential constraints such that each table in the set is a descendent of itself.
Self-referencing table	A table that is a parent and a dependent in the same referential constraint. The constraint is called a self-referencing constraint.
Self-referencing row	A row that is a parent of itself.

The purpose of a referential constraint is to guarantee that table relationships are maintained and that data entry rules are followed. This means that as long as a referential constraint is in effect, the database manager guarantees that for each row in a child table that has a non-null value in its foreign key columns, a row exists in a corresponding parent table that has a matching value in its parent key.

When an SQL operation attempts to change data in such a way that referential integrity will be compromised, a foreign key (or referential) constraint could be violated. The database manager handles these types of situations by enforcing a set of rules that are associated with each referential constraint. This set of rules consist of:

An insert rule
An update rule
A delete rule

When an SQL operation attempts to change data in such a way that referential integrity will be compromised, a referential constraint could be violated. For example,

An insert operation could attempt to add a row of data to a child table that has a value in its foreign key columns that does not match a value in the corresponding parent table's parent key.
An update operation could attempt to change the value in a child table's foreign key columns to a value that has no matching value in the corresponding parent table's parent key.
An update operation could attempt to change the value in a parent table's parent key to a value that does not have a matching value in a child table's foreign key columns.
A delete operation could attempt to remove a record from a parent table that has a matching value in a child table's foreign key columns.

The database manager handles these types of situations by enforcing a set of rules that are associated with each referential constraint. This set of rules consists of:

An insert rule
An update rule
A delete rule

Insert rule

The insert rule of a referential constraint is that a non-null insert value of the foreign key must match some value of the parent key of the parent table. The value of a composite foreign key is null if any component of the value is null. This rule is implicit when a foreign key is specified.

Update rule

The update rule of a referential constraint is specified when the referential constraint is defined. The choices are NO ACTION and RESTRICT. The update rule applies when a row of the parent or a row of the dependent table is updated.

In the case of a parent row, when a value in a column of the parent key is updated, the following rules apply:

If any row in the dependent table matches the original value of the key, the update is rejected when the update rule is RESTRICT.
If any row in the dependent table does not have a corresponding parent key when the update statement is completed (excluding AFTER triggers), the update is rejected when the update rule is NO ACTION.

The value of the parent unique keys cannot be changed if the update rule is RESTRICT and there are one or more dependent rows. However, if the update rule is NO ACTION, parent unique keys can be updated as long as every child has a parent key by the time the update statement completes. A non-null update value of a foreign key must be equal to a value of the primary key of the parent table of the relationship.

Also, the use of NO ACTION or RESTRICT as update rules for referential constraints determines when the constraint is enforced. An update rule of RESTRICT is enforced before all other constraints, including those referential constraints with modifying rules such as CASCADE or SET NULL. An update rule of NO ACTION is enforced after other referential constraints. Note that the SQLSTATE returned is different depending on whether the update rule is RESTRICT or NO ACTION.

In the case of a dependent row, the NO ACTION update rule is implicit when a foreign key is specified. NO ACTION means that a non-null update value of a foreign key must match some value of the parent key of the parent table when the update statement is completed.

The value of a composite foreign key is null if any component of the value is null.

Delete rule

The delete rule of a referential constraint is specified when the referential constraint is defined. The choices are NO ACTION, RESTRICT, CASCADE, or SET NULL. SET NULL can be specified only if some column of the foreign key allows null values.

If the identified table or the base table of the identified view is a parent, the rows selected for delete must not have any dependents in a relationship with a delete rule of RESTRICT, and the DELETE must not cascade to descendent rows that have dependents in a relationship with a delete rule of RESTRICT.

If the delete operation is not prevented by a RESTRICT delete rule, the selected rows are deleted. Any rows that are dependents of the selected rows are also affected:

The nullable columns of the foreign keys of any rows that are their dependents in a relationship with a delete rule of SET NULL are set to the null value.
Any rows that are their dependents in a relationship with a delete rule of CASCADE are also deleted, and the rules mentioned previously apply, in turn, to those rows.

The delete rule of NO ACTION is checked to enforce that any non-null foreign key refers to an existing parent row after the other referential constraints have been enforced.

The delete rule of a referential constraint applies only when a row of the parent table is deleted. More precisely, the rule applies only when a row of the parent table is the object of a delete or propagated delete operation (defined in the following section), and that row has dependents in the dependent table of the referential constraint. Consider an example where P is the parent table, D is the dependent table, and p is a parent row that is the object of a delete or propagated delete operation. The delete rule works as follows:

With RESTRICT or NO ACTION, an error occurs and no rows are deleted.
With CASCADE, the delete operation is propagated to the dependents of p in table D.
With SET NULL, each nullable column of the foreign key of each dependent of p in table D is set to null.

Any table that can be involved in a delete operation on P is said to be delete-connected to P. Thus, a table is delete-connected to table P if it is a dependent of P, or a dependent of a table to which delete operations from P cascade.

The following restrictions apply to delete-connected relationships:

When a table is delete-connected to itself in a referential cycle of more than one table, the cycle must not contain a delete rule of either RESTRICT or SET NULL.
A table must not both be a dependent table in a CASCADE relationship (self-referencing or referencing another table) and have a self-referencing relationship with a delete rule of either RESTRICT or SET NULL.
When a table is delete-connected to another table through multiple relationships where such relationships have overlapping foreign keys, these relationships must have the same delete rule and none of these can be SET NULL.
When a table is delete-connected to another table through multiple relationships where one of the relationships is specified with delete rule SET NULL, the foreign key definition of this relationship must not contain any distribution key or MDC key column, a table-partitioning key column, or RCT key column.
When two tables are delete-connected to the same table through CASCADE relationships, the two tables must not be delete-connected to each other where the delete connected paths end with delete rule RESTRICT or SET NULL.