1 Intro stored procedures Declaring parameters Using in a sproc Intro to transactions Concurrency control & recovery States of transactions Desirable properties of transactions Handling transactions Manually raising errors Primary benefits of stored procedures Intro to triggers FOR/AFTER vs INSTEAD OF clause Using triggers for data integrity rules The UPDATE() function Steen Jensen, autumn 2013

 A stored procedure (or a sproc) is a script or more correctly a batch, that is stored in the database rather than in a separate file  See example page 437 2

 Declaring a parameter requires 2 to 4pieces of information: Name Data type Default value Direction  See examples page

4  Creating output parameters, return values, dealing with errors: page 442top – 452mid

 can be used in a stored procedure to test the result of the last SQL statement  See examples page 453 – 454 5

 Think of a stored procedure, which might be useful for the Amazon example  What should the sproc contain?  Which parameter(s) should be used?  Finally try to execute the stored procedure by writing the name of the sproc followed by the necessary parameter(s) 6

 A transaction is a logical unit of database processing  A transaction includes one or more database oparations, such as INSERT, DELETE, UPDATE or SELECT  The boundaries of a trnsaction can be specified by a BEGIN TRANSACTION and a END TRANSACTION statement 7

 If concurrent transactions are executed in an uncontrolled manner, errors can occur  An example of this could be an airline seat reservation system  4 kinds of errors could occur (see Elmasri chapter 20, page 726 – 728top): The lost update problem (figure 20.3 a) The temporary update problem (figure 20.3 b) The incorrect summary problem (figure 20.3 c) The unrepeatable problem 8

 When a transaction is submitted to a DBMS for execution, the system is responsible for ensuring, that all the operations in the transaction are completed successfully, or that the transaction doesn’t have any effect  In the first case the transaction is said to be committed, whereas in the second case the transaction is aborted  6 types of failure: Computer failure (system crash) Transaction or system error (integer overflow, division by zero...) Logical errors (insufficient account balance...) Concurrency control problems (deadlocks...) Disk failure Physical problems & catastrophes 9

 The recovery manager of the DBMS keeps track of the following oparations: BEGIN TRANSACTION READ or WRITE END TRANSACTION COMMIT TRANSACTION  a successful transaction ROLLBACK TRANSACTION  an unsuccessful transaction 10

 Transaction should possess several properties, often nicknamed ACID: Atomicity: a transaction should either be performed entirely or not at all Consistency preservation: a transaction should take the database from one consistent state to another Isolation: should appear as being executed in isolation from other transactions Durability or permanency: changes applied to a database by a transaction must persist in the database 11

12  The system log, characterizing schedules: page 731 – 732bot + 733bot – 748top  And now back to the book SQL Server

 Transactions can be accepted (commit) or rejected (rollback) depending on, how the transaction went (ok or bad)  See examples page 455 – 458top 13

 By using the statement RAISERROR errors can be raised manually  See examples page

 The state parameter can be used as a place marker telling exactly where the error occurred 15

16  Error arguments, re-throwing errors, adding your own error messages: page 463bot – 468bot

 The primary benefits include: Because a sproc is stored in a database, it can be called without manually loading it. Sprocs can call other sprocs (nesting) A tool of security: users can get the needed info without having access to the underlying table(s) Can give a higher degree of performance 17

 A trigger is a special kind of stored procedure, that fires in response to a specified event  Two kinds of triggers: DDL triggers (normally only used for extreme auditing of changes/history of database structure) DML triggers  Triggers are pieces of code attached to a table or view  Trigger are automatically run – they can’t be explicitly invoked (as a sproc)  DML triggers for INSERT, DELETE, UPDATE + mix/match 18

 SQL Server puts together two working tables: Inserted (containing a copy of inserted records) and Deleted (containing a copy of deleted records)  The INSTEAD OF keyword means, that working tables are created before any constraints are checked (in contrast to FOR/AFTER)  See also figure 15-1 page

 Triggers can be used to check for integrity rules  See examples page 544 – 546mid 20

21  Using triggers for custom error messages, other trigger issues : page 547bot – 553bot

 The UPDATE function can be used to check, if a certain column has been changed  See example page 553bot – 554mid (the example is a revised version of the example from page 546) 22

23  The COLUMNS_UPDATED() function, additional about triggers: page 554mid – 558mid

 Make ex on page 558 (chapter 15) in the book without looking at the answer  Then check your answer up against the indicative answer on page 794  If possible try to implement a trigger in either the Amazon or the Adventureworks system 24