Triggers

A trigger is a special type of stored program that fires when a table is modified by an INSERT, UPDATE, or DELETE (DML) statement. Triggers implement functionality that must take place whenever a certain change occurs to the table. Because triggers are attached directly to the table, application code cannot bypass database triggers.

Typical uses of triggers include the implementation of critical business logic, the denormalization of data for performance reasons, and the auditing of changes made to a table. Triggers can be defined to fire before or after a specific DML statement executes.

In Figure 2-17, we create a trigger that fires before any INSERT statement completes against the sales table. It automatically applies free shipping and discounts to orders of a specified value.

Figure 2-17. A database trigger

Here is an explanation of the trigger definition:

Line(s)	Explanation
5	Specify the trigger name.
6	Specify that the trigger fires before an insert on the sales table.
7	Include the (currently) mandatory FOR EACH ROW clause, indicating that the statements within the trigger will be executed once for every row inserted into the sales table.
8	Use BEGIN to start the block containing statements to be executed by the trigger.
9-13	If the sale_value is greater than $500, set the value of the free_shipping column to 'Y'. Otherwise, set it to 'N'.
15-19	If the sale_value is greater than $1000, calculate a 15% discount and insert that value into the discount column. Otherwise, set the discount to 0.

The effect of the trigger is to automatically set the value of the free_shipping and discount columns. Consider the INSERT statement shown in Example 2-10.

Example 2-10. An INSERT into the sales table

INSERT INTO sales
 (customer_id, product_id, sale_date, quantity, sale_value,
 department_id, sales_rep_id)
VALUES(20,10,now( ),20,10034,4,12)

The sale is valued at $10,034 and, as such, is eligible for a 15% discount and free shipping. Example 2-11 demonstrates that the trigger correctly set these values.

Example 2-11. A trigger automatically populates the free_shipping and discount columns

mysql> SELECT sale_value,free_shipping,discount
 -> FROM sales
 -> WHERE sales_id=2500003;
+------------+---------------+----------+
| sale_value | free_shipping | discount |
+------------+---------------+----------+
| 10034 | Y | 1505 |
+------------+---------------+----------+
1 row in set (0.22 sec)

Using a trigger to maintain the free_shipping and discount columns ensures that the columns are correctly maintained regardless of the SQL statements that might be executed from PHP , C#, or Java, or even from the MySQL command-line client.