Triggers and Transactions

Making Decisions behind the scene

Overview

• Majority of databases are used behind some form of front-end application

• Applications can be written to contain DBMS rules for data flow and storage

• Allowing DBMS to administer rules is more efficient and less costly when changes to rules are required

• DBMS can act on data when it is received from host application

What Controls the Flow?

Application contains code to maintain Data Integrity.

Additional point of failure created.

Application gathers and sends raw data to DBMS

DBMS applies data to tables based on

standard rules

DBMS tests data against User defined rules and

makes decision on when/how data is

processed

Data Integrity may be at risk due

additional point of failure

Data Integrity maintained

DBMS

DBMS

Triggers

• Often used to maintain Data Integrity. Also used to perform tests on business rules (eg: customer credit limits)

• External rules designed to test the effect Inserting, Updating and Deleting data will have on data tables

• Can manipulate data based on outcome of the test• Can be applied before or after data tables have been altered• Code stored in DBMS and will automatically activate when a pre-

defined event occurs (data inserted into table)• Generally execute faster than front-end application code• Triggers can only handle one type of operation at a time (Insert,

Update or Delete).• Triggers do not work on Views (or temporary tables)

Triggers

Sales_ID Cust_ID sales sale_date paid

200 A132 250 Jan 5 2009 250

201 C234 600 Jan 7 2009 600

202 A132 500 Jan 8 2009 0

cust_ID limit

A132 900

C234 600

D567 1200

Table: sales

Table: cust_cred

Note: Customer A132 is about to make a new purchase for $300.

Create Trigger cust_cred_limitBefore Insert on salesFor Each Row

BeginDeclare tot_owing as IntegerDeclare cred_limit as Integer

Select Sum(sales) into tot_owing From Sales Where cust_ID = “A132” and paid =0;

Select limit From cust_cred INTO cred_limit Where cust_ID = “A132”;

If cred_limit > tot_owing Then Insert Into Sales(sales_ID, cust_ID, sales, sale_date, paid);End If;End$$

Creating TriggersTrigger will activate before data is added to the sales table. What would happen if ‘Before’ was changed to ‘After’?

Set-up variables to hold temporary data.

Total the sales for the customer, but only those invoices that have not been paid. Place calculated value into variable

Get the credit limit value for the customer

Test to see if the customer has not exceeded their credit limit. If not then add the sale to the table

Note: If the credit limit has been exceeded, the sale was not added.

Transactions

• Transactions are extremely important, in terms of data integrity

• Transactions vital when data affects multiple tables• Determines if tables will be altered or queries will be aborted

due to some failure (power, code violation, etc)Transaction KeywordsBegin - Starts transaction process. All subsequent queries will be executed,

but data will not be permanently written to tablesCommit - Tells system to make all query alterations permanent.Rollback – Tells system to remove alterations from the tables. (Note: If the

Commit command has already been executed, Rollback will not work)

Transactions

Pending Queries

1) Add sale to ‘sales’ table2) Add commission to ‘commissions table’. This requires a calculation

(sales x commission %)

Both queries must be successful to complete the transaction

TransactionsMysql: mysql> BEGIN;

mysql> Insert Into Sales Values(sales_ID, cust_ID, sales, sale_date, paid, emp_ID);

Mysql> Insert Into commissions Values(sales_ID, emp_ID, comm_amt);

mysql> ROLLBACK;

Without a COMMIT statement, the insert was not permanent, and was reversed with the ROLLBACK. Note that the added record was visible during the transaction from the same user account that created it. No other user would see the changes until transaction was committed.

Mysql: mysql> BEGIN;

mysql> Insert Into Sales Values(sales_ID, cust_ID, sales, sale_date, paid, emp_ID);

Mysql> Insert Into commissions Values(sales_ID, emp_ID, comm_amt);

mysql> Commit;

With a COMMIT statement, the insert becomes permanent. All other users can now see changes.

TransactionsWhat happens if the the computer fails(power failure, etc.) in the middle of

transaction?

mysql> BEGIN;

mysql> Insert Into Sales Values(sales_ID, cust_ID, sales, sale_date, paid, emp_ID);

Mysql> Insert Into commissions Values(sales_ID, emp_ID, comm_amt);

mysql> Commit;

Power Failure!!

Because the Commit command was never executed, all queries are automatically reversed. Without a Transaction, data integrity would have been lost.

TransactionsYou can test Transactions from the Command Line

By default MySQL automatically commits queries that are not part of a transaction.

To change this, type the following;

SET AUTOCOMMIT = O;

Now, all queries will react as if you had typed BEGIN, to start a transaction. To complete your queries you have to type COMMIT;

To turn AUTOCOMMIT back on, type;

SET AUTOCOMMIT = 1;

Now your queries will function, as before.