142 CHAPTER 7: HOW TO USE VIEWS ON DELETE CASCADE, start_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL, end_date TIMESTAMP CHECK (start_date <= end_date), job_type INTEGER DEFAULT 99 NOT NULL CHECK (job_type = 99)); CREATE TABLE Job_not99_Assignments (store_nbr INTEGER NOT NULL REFERENCES Stores (store_nbr) ON UPDATE CASCADE ON DELETE CASCADE, ssn CHAR(9) NOT NULL PRIMARY KEY REFERENCES Personnel (ssn) ON UPDATE CASCADE ON DELETE CASCADE, start_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL, end_date TIMESTAMP CHECK (start_date <= end_date), job_type INTEGER DEFAULT 0 NOT NULL CHECK (job_type BETWEEN 0 AND 98)—no 99 code ); Then build a UNION-ed VIEW: CREATE VIEW JobAssignments (store_nbr, ssn, start_date, end_date, job_type) AS (SELECT store_nbr, ssn, start_date, end_date, job_type FROM Job_not99_Assignments UNION ALL SELECT store_nbr, ssn, start_date, end_date, job_type FROM Job_99_Assignments) The key and job_type constraints in each table working together will guarantee only one manager per store. The next step is to add INSTEAD OF triggers to the VIEW, so that the users can insert, update, and delete from it easily. As an exercise for the reader: How would you ensure that no store has more than two assistant managers? 7.8 Updatable VIEWs 143 7.8 Updatable VIEWs The SQL-92 standard is actually conservative about which VIEWs are updatable. They have to be based on the following: 1. A SELECT statement on one and only one table, but the VIEW can be defined on several layers of VIEWs on top of VIEWs. 2. The VIEW must include all of the columns of a UNIQUE or PRIMARY KEY constraint in the base table. This guarantees that all of the rows in the VIEW map back to one and only one row in the base table from which it is derived. 3. All base table columns not shown in the VIEW must have default values or be NULL-able. The reason for that is obvious: You have to delete or insert a complete row into the base table, so the system must be able to construct such a row. However, other VIEWs are updatable, and some vendors support more than the basic version given in the SQL-92 standard. The VIEW must have an INSERT, UPDATE, and DELETE rule under the covers, which maps its rows back to a single row in the base table(s). 7.8.1 WITH CHECK OPTION clause Another feature, which is not used enough, is the WITH CHECK OPTION clause on a VIEW. It is a bit tricky, when you nest VIEWs inside each other, but the idea is that an UPDATE or INSERT INTO statement cannot leave the scope of the set selected by the updatable VIEW. For example, we have a VIEW like this: CREATE VIEW NewYorkSalesmen (ssn, name, ) AS SELECT ssn, name, FROM Salesmen WHERE city = 'New York'; And we update it, thus: UPDATE NewYorkSalesmen SET city = 'Boston'; 144 CHAPTER 7: HOW TO USE VIEWS The result would be that “NewYorkSalesmen” would be empty when you come back to it. This is probably not desirable. However, if we had defined the updatable VIEW as: CREATE VIEW NewYorkSalesmen (ssn, name, ) AS SELECT ssn, name, FROM Salesmen WHERE city = 'New York' WITH CHECK OPTION; the system would test the update for a violation and would reject it. 7.8.2 INSTEAD OF Triggers Because some VIEWs cannot be updated, you can add INSTEAD OF triggers to fool the users. This trigger is executed instead of the INSERT, UPDATE, or DELETE action, thus overriding the actions of the triggering statements. The syntax will vary from product to product, but expect something like this: CREATE TRIGGER <trigger name> ON <table name > [BEFORE | AFTER | INSTEAD OF] [INSERT| DELETE | UPDATE] AS [<sql stmt> | BEGIN ATOMIC {<sql stmt>;} END] For obvious reasons, only one INSTEAD OF trigger per INSERT, UPDATE, or DELETE statement can be defined on a table or VIEW. However, it is possible to define VIEWs on VIEWs where each VIEW has its own INSTEAD OF trigger. INSTEAD OF triggers are not allowed on updatable VIEWs that have a WITH CHECK OPTION. You can also define INSTEAD OF triggers on base tables, but this is a bit weird because you have BEFORE and AFTER triggers. 7.9 Have a Reason for Each VIEW Rationale: VIEWs should be created only when they achieve a specific, reasonable goal. Each VIEW should have a specific application or business requirement that it fulfills before it is created. That requirement should 7.11 Synchronize VIEWs with Base Tables 145 be documented somewhere, preferably in a data dictionary or possibly as a remark in the VIEW declaration. Exceptions: None 7.10 Avoid VIEW Proliferation Rationale: The proliferation avoidance rule is based on common sense. Why create something that is not needed? It just takes up space that could be used for something that is needed. Whenever a SQL object is created, additional entries are placed in the schema information tables. Creating needless schema objects causes what Craig Mullins calls catalog clutter . For example, in DB2, every unnecessary VIEW that is created in SQL will potentially insert rows into four VIEW-specific schema information tables (i.e., SYSVTREE, SYSVLTREE, SYSVIEWS, and SYSVIEWDEP) and three table-specific schema information tables (i.e., SYSTABLES, SYSTABAUTH, and SYSCOLUMNS). It is a good idea to use a utility program to see if you have VIEWs that are not referenced anywhere. Another good idea is to see if you have VIEWs that do the same thing, or almost the same thing, so you can remove one of them. Exceptions: None 7.11 Synchronize VIEWs with Base Tables Rationale: Whenever a base table changes, all VIEWs that depend on that base table should be analyzed to determine if the change affects them. All VIEWs should remain logically pure. The VIEW should remain useful for the specific reason you created it. For example, say a VIEW was created to control employee access to a project and we add the new badge numbers to the Personnel table. This badge number probably should also be added to the access VIEW. The badge number column can be added to the Personnel table immediately and then to the VIEW at the earliest convenience of the development team. 146 CHAPTER 7: HOW TO USE VIEWS The synchronization rule requires that strict change impact analysis procedures be in place. Every change to a base table should trigger the usage of these utility programs and maintenance procedures. Exceptions: None 7.12 Improper Use of VIEWs Over the years, VIEWs have been used for other purposes that made sense at the time but have been rendered obsolete with the advent of new DBMS functionality. 7.12.1 VIEWs for Domain Support Rationale: It is a sad fact of life that most RDBMS do not support domains. Domains were in the original relational model and should have been part of SQL from the start. A domain basically identifies the valid range of values that a column can contain. Of course, domains are more complex than this simple explanation. For example, only columns pooled from the same domain should be able to be compared within a predicate (unless explicitly overridden). Some of the functionality of domains can be implemented using VIEWs and the WITH CHECK OPTION clause, which ensures the update integrity of VIEWs. This will guarantee that all data inserted or updated using the VIEW will adhere to the VIEW specification. CREATE VIEW Personnel (ssn, name, sex, ) AS SELECT ssn, name, sex, FROM ISBN0008 –- a name you did not want anyone to see WHERE sex IN (0, 1, 2) –- iso codes WITH CHECK OPTION; Now, this method of using VIEWs to simulate domains is still viable, but a better technique to provide the same functionality is available— namely, CHECK() constraints. CREATE TABLE Personnel (ssn CHAR(9) NOT NULL, name, sex, ) . <table name > [BEFORE | AFTER | INSTEAD OF] [INSERT| DELETE | UPDATE] AS [< ;sql stmt> | BEGIN ATOMIC {< ;sql stmt>;} END] For obvious reasons, only one INSTEAD OF trigger per INSERT,. row. However, other VIEWs are updatable, and some vendors support more than the basic version given in the SQL- 92 standard. The VIEW must have an INSERT, UPDATE, and DELETE rule under the covers, which. store has more than two assistant managers? 7.8 Updatable VIEWs 143 7.8 Updatable VIEWs The SQL- 92 standard is actually conservative about which VIEWs are updatable. They have to be based