Structured Query Language (SQL) - Cultural View of Technology

Foreign key 55
SET NULL
The foreign key values in the referencing row are set to NULL when the referenced row is updated or deleted. This is
only possible if the respective columns in the referencing table are nullable. Due to the semantics of NULL, a
referencing row with NULLs in the foreign key columns does not require a referenced row.
SET DEFAULT
Similarly to SET NULL, the foreign key values in the referencing row are set to the column default when the
referenced row is updated or deleted.
Triggers
Referential actions are generally implemented as implied triggers (i.e. triggers with system-generated names, often
hidden.) As such, they are subject to the same limitations as user-defined triggers, and their order of execution
relative to other triggers may need to be considered; in some cases it may become necessary to replace the referential
action with its equivalent user-defined trigger to ensure proper execution order, or to work around mutating-table
limitations.
Another important limitation appears with transaction isolation: your changes to a row may not be able to fully
cascade because the row is referenced by data your transaction cannot "see", and therefore cannot cascade onto. An
example: while your transaction is attempting to renumber a customer account, a simultaneous transaction is
attempting to create a new invoice for that same customer; while a CASCADE rule may fix all the invoice rows your
transaction can see to keep them consistent with the renumbered customer row, it won't reach into another
transaction to fix the data there; because the database cannot guarantee consistent data when the two transactions
commit, one of them will be forced to rollback (often on a first-come-first-served basis.)
Example
As a first example to illustrate foreign keys, suppose an accounts database has a table with invoices and each invoice
is associated with a particular supplier. Supplier details (such as address or phone number) are kept in a separate
table; each supplier is given a 'supplier number' to identify it. Each invoice record has an attribute containing the
supplier number for that invoice. Then, the 'supplier number' is the primary key in the Supplier table. The foreign
key in the Invoices table points to that primary key. The relational schema is the following. Primary keys are marked
in bold, and foreign keys are marked in italics.
Supplier ( SupplierNumber, Name, Address, Type )
Invoices ( InvoiceNumber, SupplierNumber, Text )
The corresponding Data Definition Language statement is as follows.
CREATE TABLE Supplier (
SupplierNumber INTEGER NOT NULL,
Name VARCHAR(20) NOT NULL,
Address VARCHAR(50) NOT NULL,
Type VARCHAR(10),
CONSTRAINT supplier_pk PRIMARY KEY(SupplierNumber),
CONSTRAINT number_value CHECK (SupplierNumber > 0) )
CREATE TABLE Invoices (
InvoiceNumber INTEGER NOT NULL,
SupplierNumber INTEGER NOT NULL,