Procedures and triggers in SQL

Outlines of Session
· Why Procedures in MySQL/MSSQL
· Advantages
· Syntax
· LIVE creation & Execution of procedures
· DB connection with MSSQL
· Procedures VS Functions
· Triggers : Usage & implementation
· LIVE creation & Execution of Database triggers

Stored Procedures in MySQL
A stored procedure contains a
sequence of SQL commands stored in
the database catalog so that it can be
invoked later by a program

Procedures in MySQL - Why?
Why go to the trouble of extracting logic
from your application, putting it into a
different format, and placing it on the
database server?

Procedures in MySQL - Advantages
Advantages:
● FASTER in general than using app program
● Improve the security of server
● Not subject to SQL injection attacks.
● Encapsulation of business logic
● Portable
● Server Overhead
● Avoidance of network traffic

Procedures in MySQL - Declaration
● Stored procedures are declared using the following syntax:
Create Procedure <proc-name>
(param_spec1, param_spec2, …, param_specn )
begin
-- execution code
end;
where each param_spec is of the form:
[in | out | inout] <param_name> <param_type>
– in mode: allows you to pass values into the procedure,
– out mode: allows you to pass value back from procedure to the calling
program

Procedures in MySQL - Example 1
mysql> delimiter //
mysql> CREATE PROCEDURE dorepeat(p1 INT)
-> BEGIN
-> SET @x = 0;
-> REPEAT SET @x = @x + 1;
-> UNTIL @x > p1;
-> END REPEAT;
-> END
-> //
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql> CALL dorepeat(1000);
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT @x;
+------+
| @x |
+------+
| 1001 |
+------+
1 row in set (0.00 sec)
SET @x = 0;
REPEAT SET @x = 0 + 1;
UNTIL 1 > 1000
END REPEAT;

Procedures in MySQL - Example 2
DELIMITER $$
CREATE PROCEDURE CountPartByType
(IN PartType VARCHAR(2),
OUT total INTEGER)
BEGIN
SELECT count(Part_Num) INTO total
FROM PART
WHERE Class = PartType;
END$$
DELIMITER ;
Call CountPartByType ('HW',@total); //in command line
SELECT @total;
$result = mysql_query('CALL CountPartByType('HW',@total)'); //in php

Connecting MS SQL - In Cake
Path : /app/Config/database.php
The name of a supported datasource; valid options are as follows:
* Database/Mysql - MySQL 4 & 5,
* Database/Sqlite - SQLite (PHP5 only),
* Database/Postgres - PostgreSQL 7 and higher,
* Database/Sqlserver - Microsoft SQL Server 2005 and higher
Eg.
public $default = array(
'datasource' => 'Database/Sqlserver',
'persistent' => false,
'host' => 'localhost',
'login' => '**********',
'password' => '**********',
'database' => '**********',
'prefix' => '',
'encoding' => 'utf8',
);

Functions V/S Procedures MySQL
Functions Procedures
could be used in SELECT statements,
provided they don’t do any data
manipulation.
cannot be included in SELECT statements.
must return a value (using the RETURN
keyword), but for stored procedures this is
not compulsory.
can use RETURN keyword but without any
value being passed.A stored procedure can
return multiple values using the OUT
parameter or return no value at all.
A function is a subprogram written to
perform certain computations
A scalar function returns only a single value
(or NULL), whereas a table function returns
a (relational) table comprising zero or more
rows, each row with one or more columns.
Its not a DB object. A stored procedure is a database object.
A stored procedure saves the query
compilation time.

Example
● Suppose we want to keep track of the total salaries of employees
working for each department
We need to write a procedure
to update the salaries in
the deptsal table

Example
Step 1: Change the delimiter (i.e., terminating character) of
SQL statement from semicolon (;) to something else (e.g., //)
So that you can distinguish between the semicolon of the SQL
statements in the procedure and the terminating character of
the procedure definition

Example
Step 2:
1. Define a procedure called updateSalary which takes as
input a department number.
2. The body of the procedure is an SQL command to update
the totalsalary column of the deptsal table.
3. Terminate the procedure definition using the delimiter you
had defined in step 1 (//)

Example
Step 3: Change the delimiter back to semicolon (;)

Example
Step 4: Call the procedure to update the totalsalary for each
department

Example
Step 5: Show the updated total salary in the deptsal table

● Use show procedure status to display the list of stored
procedures you have created
● Use drop procedure to remove a stored procedure

● You can declare variables in stored procedures
● You can use flow control statements (conditional IF-THEN-
ELSE or loops such as WHILE and REPEAT)

Another Example
● Create a procedure to give a raise to all employees

SQL Triggers
● To monitor a database and take a corrective action when a condition
occurs
– Examples:
◆ Charge $10 overdraft fee if the balance of an account after a withdrawal
transaction is less than $500
◆ Limit the salary increase of an employee to no more than 5% raise
CREATE TRIGGER trigger-name
trigger-time trigger-event
ON table-name
FOR EACH ROW
trigger-action;
– trigger-time ∈ {BEFORE, AFTER}
– trigger-event ∈ {INSERT,DELETE,UPDATE}

SQL Triggers: An Example
● We want to create a trigger to update the total salary of a
department when a new employee is hired

● Create a trigger to update the total salary of a department
when a new employee is hired:
● The keyword “new” refers to the new row inserted

totalsalary increases by 90K
totalsalary did not change

● A trigger to update the total salary of a department when an
employee tuple is modified(insert/delete):

● A trigger to update the total salary of a department when an
employee tuple is deleted:

SQL Triggers
● To list all the triggers you have created:
mysql> show triggers;

Procedures and triggers in SQL

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