Structured Query Language (SQL) - Cultural View of Technology

Foreign key 54
CONSTRAINT col3_fk FOREIGN KEY(col3)
... )
REFERENCES other_table(key_col) ON DELETE CASCADE,
If the foreign key is a single column only, the column can be marked as such using the following syntax:
CREATE TABLE table_name (
id INTEGER PRIMARY KEY,
col2 CHARACTER VARYING(20),
col3 INTEGER REFERENCES other_table(column_name),
... )
Referential Actions
Because the Database Management System enforces referential constraints, it must ensure data integrity if rows in a
referenced table are to be deleted (or updated). If dependent rows in referencing tables still exist, those references
have to be considered. SQL:2003 specifies 5 different referential actions that shall take place in such occurrences:
• CASCADE
• RESTRICT
• NO ACTION
• SET NULL
• SET DEFAULT
CASCADE
Whenever rows in the master (referenced) table are deleted, the respective rows of the child (referencing) table with
a matching foreign key column will get deleted as well. This is called a cascade delete.
Example Tables: Customer(customer_id, cname, caddress) and Order(customer_id, products, payment)
Customer is the master table and Order is the child table, where 'customer_id' is the foreign key in Order and
represents the customer who placed the order. When a row of Customer is deleted, any Order row matching the
deleted Customer's customer_id will also be deleted.
RESTRICT
A value cannot be updated or deleted when a row exists in a foreign key table that references the value in the
referenced table.
Similarly, a row cannot be deleted as long as there is a reference to it from a foreign key table.
NO ACTION
NO ACTION and RESTRICT are very much alike. The main difference between NO ACTION and RESTRICT is
that with NO ACTION the referential integrity check is done after trying to alter the table. RESTRICT does the
check before trying to execute the UPDATE or DELETE statement. Both referential actions act the same if the
referential integrity check fails: the UPDATE or DELETE statement will result in an error.
In other words, when an UPDATE or DELETE statement is executed on the referenced table using the referential
action NO ACTION, the DBMS verifies at the end of the statement execution that none of the referential
relationships are violated. This is different from RESTRICT, which assumes at the outset that the operation will
violate the constraint. Using NO ACTION, the triggers or the semantics of the statement itself may yield an end state
in which no foreign key relationships are violated by the time the constraint is finally checked, thus allowing the
statement to complete successfully.