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6 Integrity and Security
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The McGraw Hill Companies, 2001
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create table customer (customer-name char(20), customer-street char(30), customer-city char(30), primary key (customer-name)) create table branch (branch-name char(15), branch-city char(30), assets integer, primary key (branch-name), check (assets >= 0)) create table account (account-number char(10), branch-name char(15), balance integer, primary key (account-number), foreign key (branch-name) references branch, check (balance >= 0)) create table depositor (customer-name char(20), account-number char(10), primary key (customer-name, account-number), foreign key (customer-name) references customer, foreign key (account-number) references account) Figure 62
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account relation, deleting the tuple that refers to the branch that was deleted Similarly, the system does not reject an update to a eld referenced by the constraint if it violates the constraint; instead, the system updates the eld branch-name in the referencing tuples in account to the new value as well SQL also allows the foreign key clause to specify actions other than cascade, if the constraint is violated: The referencing eld (here, branch-name) can be set to null (by using set null in place of cascade), or to the default value for the domain (by using set default) If there is a chain of foreign-key dependencies across multiple relations, a deletion or update at one end of the chain can propagate across the entire chain An interesting case where the foreign key constraint on a relation references the same relation appears in Exercise 64 If a cascading update or delete causes a constraint violation that cannot be handled by a further cascading operation, the system aborts the transaction As a result, all the changes caused by the transaction and its cascading actions are undone Null values complicate the semantics of referential integrity constraints in SQL Attributes of foreign keys are allowed to be null, provided that they have not other-
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6 Integrity and Security
The McGraw Hill Companies, 2001
6
Integrity and Security
wise been declared to be non-null If all the columns of a foreign key are non-null in a given tuple, the usual de nition of foreign-key constraints is used for that tuple If any of the foreign-key columns is null, the tuple is de ned automatically to satisfy the constraint This de nition may not always be the right choice, so SQL also provides constructs that allow you to change the behavior with null values; we do not discuss the constructs here To avoid such complexity, it is best to ensure that all columns of a foreign key speci cation are declared to be non-null Transactions may consist of several steps, and integrity constraints may be violated temporarily after one step, but a later step may remove the violation For instance, suppose we have a relation marriedperson with primary key name, and an attribute spouse, and suppose that spouse is a foreign key on marriedperson That is, the constraint says that the spouse attribute must contain a name that is present in the person table Suppose we wish to note the fact that John and Mary are married to each other by inserting two tuples, one for John and one for Mary, in the above relation The insertion of the rst tuple would violate the foreign key constraint, regardless of which of the two tuples is inserted rst After the second tuple is inserted the foreign key constraint would hold again To handle such situations, integrity constraints are checked at the end of a transaction, and not at intermediate steps1
63 Assertions
An assertion is a predicate expressing a condition that we wish the database always to satisfy Domain constraints and referential-integrity constraints are special forms of assertions We have paid substantial attention to these forms of assertion because they are easily tested and apply to a wide range of database applications However, there are many constraints that we cannot express by using only these special forms Two examples of such constraints are: The sum of all loan amounts for each branch must be less than the sum of all account balances at the branch Every loan has at least one customer who maintains an account with a minimum balance of $100000 An assertion in SQL takes the form create assertion <assertion-name> check <predicate> Here is how the two examples of constraints can be written Since SQL does not provide a for all X, P (X) construct (where P is a predicate), we are forced to im1 We can work around the problem in the above example in another way, if the spouse attribute can be set to null: We set the spouse attributes to null when inserting the tuples for John and Mary, and we update them later However, this technique is rather messy, and does not work if the attributes cannot be set to null
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