1. The SQL Query Language DML1 The SQL Query Language DML Odds and Ends

2. The SQL Query Language DML2 Outline  Data creation and destruction  Inserting into a table  Deleting from a table  Modifying values in a table  Other commonly used features  Views  Transactions and triggers  Summary

3. The SQL Query Language DML3 Modifications  There are three modification statements  INSERT  UPDATE  DELETE  For insertions, either values can be specified, or a select statement provides the values  Enter a reservation for Eric for the film 332244 INSERT INTO Reserved VALUES (123456, 332244, CURRENT_DATE)

4. The SQL Query Language DML4 INSERT, cont.  Let Melanie reserve all the films that Eric has reserved. INSERT INTO Reserved SELECT C2.CustomerID, FilmID, CURRENT_DATE FROM Reserved, Customer C, Customer C2 WHERE C.Name = ’Eric’ AND C.CustomerID = Reserved.CustomerID AND C2.Name = ’Melanie’

5. The SQL Query Language DML5 DELETE  A where clause identifies which rows to remove from the table  Delete all the reservations of customer 123456 DELETE FROM Reserved WHERE CustomerID = 123456  Other tables can be consulted to determine which rows should be removed  Delete all of Eric's reservations DELETE FROM Reserved WHERE CustomerID = ANY(SELECT CustomerID FROMCustomer WHERE Name = ’Eric’)

6. The SQL Query Language DML6 DELETE, cont  Delete the films with a rental price below the average DELETE FROM Film WHERE RentalPrice <(SELECT AVG(RentalPrice) FROM Film)  Implementation difficulty: As we delete rows from Film, the average rental balance changes  Not a problem Inner query, first computes the average balance Outer “loop” deletes tuples without recomputing AVG

7. The SQL Query Language DML7 UPDATE  Increase the rental price of all films by 10% UPDATE Film SET RentalPrice = RentalPrice * 1.10  The update statement has an optional where clause  Increase the rental price of foreign films by 10% UPDATE Film SET RentalPrice = RentalPrice * 1.10 WHERE Kind = ’F’

8. The SQL Query Language DML8 UPDATE, cont  Increase the rental price of films with a current price of under $3 by 20%, and all others by 10% UPDATE Film SET RentalPrice = RentalPrice * 1.10 WHERE RentalPrice >= 3.00 UPDATE Film SET RentalPrice = RentalPrice * 1.20 WHERE RentalPrice < 3.00  Careful, order is important!

9. The SQL Query Language DML9 Outline  Data creation and destruction  Inserting into a table  Deleting from a table  Modifying values in a table  Other commonly used features  Views  Transactions and triggers  Summary

10. The SQL Query Language DML10 View  Views provide a mechanism to create a virtual table CREATE VIEW name AS query expression  To create a view we use the command  Define a view of all customers in Dublin CREATE VIEW Dublin_Customers AS SELECT * FROM Customer WHERE City = ’Dublin’

11. The SQL Query Language DML11 View, cont.  Define a view of all customers holding reservations, and the films they have reserved CREATE VIEW Reservations AS SELECT Name, Title FROM Customer, Reserved, Film WHERE Customer.CustomerID = Reserved.CustomerID AND Reserved.FilmID = Film.FilmID

12. The SQL Query Language DML12 Transactions  A transaction identifies an elementary unit of work carried out by an application, to which we wish to allocate particular characteristics of reliability and isolation  A system that makes available mechanisms for the definition and execution of transactions is called a transaction processing system  Transactions are initiated with any SQL statement that modifies the database

13. The SQL Query Language DML13 Transactions  A transaction can be defined syntactically: each transaction, irrespective of the language in which it is written, is enclosed whthin two commands begin transaction end transaction  Within the transaction code, two particular instructions can appear commit work rollback work

14. The SQL Query Language DML14 Transaction example  An example of transaction is given in the following code begin transaction X := x – 10; Y := y + 10; Commit work; end transaction

15. The SQL Query Language DML15 Active databases  An active database system is a DBMS that supports an integrated subsystem for the definition and management of production rules  The rules follow the event – condition – action paradigm: each rule reacts to some events, evaluates a condition and, based on the truth value of the condition, might carry out an action  The execution of the rules happens under the control of an autonomous subsystem, known as the rule engine, which keeps track of the events that have occurred and schedules the rules for execution  One of the active rules called triggers

16. The SQL Query Language DML16 Triggers  The creation of triggers is part of the DDL  Maintain data integrity  Associated with a table (view)  Event-condition-action  Wait for a table event  On event, evaluate condition  If condition is true, execute action X before after insertion deletion update

17. The SQL Query Language DML17 How to create a trigger (SQL Server Enterprise Manager) 1.Expand a server group, and then expand a server. 2.Expand Databases, expand the database in which the table to contain the trigger belongs, and then click Tables. 3.In the details pane, right-click the table on which the trigger will be created, point to All Tasks, and then click Manage Triggers. 4.In Name, click. 5.In the Text box, enter the text of the trigger. Use CTRL-TAB to indent the text of a trigger. 6.To check the syntax, click Check Syntax.

18. The SQL Query Language DML18 Syntax (SQL server) CREATE TRIGGER trigger_name ON { table | view } [ WITH ENCRYPTION ] { { { FOR | AFTER | INSTEAD OF } { [ INSERT ] [, ] [ UPDATE ] } [ WITH APPEND ] [ NOT FOR REPLICATION ] AS [ { IF UPDATE ( column ) [ { AND | OR } UPDATE ( column ) ] [...n ] | IF ( COLUMNS_UPDATED ( ) { bitwise_operator } updated_bitmask ) { comparison_operator } column_bitmask [...n ] } ] sql_statement [...n ] } }

19. The SQL Query Language DML19 Oracle Table-level Trigger Body  Example: At most 100 people can live in Dublin  After update/insert into Person DECLARE Declare Variables C INTEGER; BEGIN Body of Trigger SELECT COUNT(Name) INTO C FROM Person WHERE City = ‘Dublin’; IF (C > 100) THEN RAISE_APPLICATION_ERROR(-20000, ‘too many in Dublin’); END IF; END

20. The SQL Query Language DML20 Potential Applications  Notification  an active database may be used to monitor  Enforce integrity constraints  Business roles  Maintenance of derived data  Maintain the derived attribute whenever individual tuples are changed

21. The SQL Query Language DML21 Trigger Gotchas  Potentially infinite loop  Trigger A: On insertion into Person, insert into Population  Trigger B: On insertion into Population, insert into Person  Mutating tables  Trigger A: On insertion into Person, insert into Person!  Disallowed!  Trigger cannot make changes to table that trigger is defined on

22. The SQL Query Language DML22 Summary  Schema Definition  CREATE TABLE  CREATE VIEW  ALTER  DROP  Queries  SELECT  Modifications  INSERT  DELETE  UPDATE  Transaction Management  COMMIT  ROLLBACK  Active Database  Trigger