Assorted Topics Stored Procedures and Triggers Pg 1

Size: px

Start display at page:

Download "Assorted Topics Stored Procedures and Triggers Pg 1"

Kenneth Dwayne Parrish
5 years ago
Views:

1 Assorted Topics Stored Procedures and Triggers Pg 1 Stored Procedures and Triggers Ray Lockwood Points: A Stored Procedure is a user-written program stored in the database. A Trigger is a stored procedure that s automatically executed whenever a specified event occurs. The Stored Procedure A Stored Procedure is a small user-written program that s stored as a database object. There s no standard language for writing a stored procedure. Oracle, for example, supports Java and PL/SQL (a proprietary Procedural Language). Microsoft SQL Server supports the.net languages and its proprietary Transact-SQL. MySQL also uses a proprietary language for stored procedures. Here s a sample MySQL stored procedure taken from the MySQL documentation found here: MySQL only recently (ver 5.0) implemented stored procedures & triggers. These features are not as highly developed as in other DBMS s. SET variable1 = 'birds'; /* assignment */ If you want to use more than one line of code, use the BEGIN and END block delimiters. As you can see, a stored procedure can accept calling parameters and execute SQL statements. It can also return a value, in which case, it s called a Stored Function. A Stored Procedure resides in and is attached to a particular database. It can be executed by any process using the database as long as the process has the correct permission.

2 Assorted Topics Stored Procedures and Triggers Pg 2 Stored Procedure Syntax The DELIMITER Keyword There s just one little trick to writing a stored procedure: the DELIMITER keyword. The example above contains complete SQL statements, each of which ends with a semicolon. However, if semicolons work the way they always do, the the first one encountered would terminate the CREATE PROCEDURE statement: All these semicolons must somehow be saved as part of the Stored Procedure. CREATE statement SET variable1 = 'birds'; ends right here! /* The assignment */ rest of the procedure is lost. The semicolon is the SQL statement delimiter. To get the semicolons saved as part of the stored procedure, we ll have to change a syntax rule. We use the DELIMITER keyword to change the delimiter from the semicolon to something else: Change the SQL statement delimiter from the semicolon to $$ Traditionally, we choose $$ as a temporary statement delimiter when writing a stored procedure because it s unlikely to conflict. SET variable1 = 'birds'; /* assignment */ Since the semicolon is no longer the delimiter, the CREATE statement doesn t end here. The CREATE statement ends here. Don t forget to set the delimiter back to the semicolon!

3 Assorted Topics Stored Procedures and Triggers Pg 3 Calling a Stored Procedure A Stored Procedure can be called from application code written in C++, Java,.NET, or any language with an interface to the DBMS. You can call a Stored Procedure from the command line by using a CALL statement: CALL storedproc(parameters); You can see the return values from a Stored Function by using the SELECT statement: SELECT storedfunc(parameters); Uses for Stored Procedures Reduce Network Traffic. Stored procedures aren't distributed to the client computers where application code runs. They stay on and run in the database server. Therefore, interaction between the database and a stored procedure occurs within the server and not over the network. This increases speed and reduces network demand. A Stored Function is a Stored Procedure that returns a value. Use the CREATE FUNCTION statement to declare a stored function. Enhance Security. All the SQL statements that interact with the database can be saved in stored procedures. Stored procedures can be used as database setters and getters, similar to those in object oriented programming. This eliminates the need for SQL to be embedded in application code, enabling better control over the queries that interact with the database. Prevent SQL Injection Attacks. A SQL injection attack occurs when a malicious user enters snippets of SQL into the fields of a user interface. If the user interface concatenates these snippets into a SQL statement, the SQL snippets will change the statement s operation. Instead, a stored procedure can accept user input via its calling parameters, and examine each one before using it in a SQL statement. Standardize Processing. Application logic can be placed into stored procedures. This ensures that all database applications use the same logic. In the past it was considered a bad practice to put logic into stored procedures, but current thought has reversed. The Trigger A Trigger is a type of stored procedure that's executed whenever a specified event occurs. This differs from a regular stored procedure which is explicitly called. A trigger is attached to a table or a view. (This contrasts to a stored procedure which is attached to a database). A trigger can be attached to only a single table or a single view, but a table or view may have many triggers. Most DBMS s use the same programming language in triggers as in stored procedures. allow a trigger to be associated with a view.

4 Assorted Topics Stored Procedures and Triggers Pg 4 The purpose of a trigger is to assist during changes to the data in a table or view. This means that three types of SQL statements will fire a trigger: 1. INSERT 2. DELETE 3. UPDATE There are three kinds of triggers: 1. BEFORE fires before the SQL statement executes 2. INSTEAD OF fires in place of the SQL statement 3. AFTER fires after the SQL statement executes support INSTEAD OF triggers. Therefore, there are nine types of triggers: 1-3 BEFORE (INSERT, UPDATE, DELETE) 4-6 AFTER (INSERT, UPDATE, DELETE) 7-9 INSTEAD OF (INSERT, UPDATE, DELETE). Trigger Syntax This is the syntax to create a MySQL trigger: CREATE TRIGGER <trigger name> { BEFORE AFTER } { INSERT UPDATE DELETE } ON <table name> FOR EACH ROW <triggered SQL statement> Here s an example from the MySQL documentation: CREATE TRIGGER testref BEFORE INSERT Attach the trigger to this table. ON test1 FOR EACH ROW BEGIN INSERT INTO test2 SET a2 = NEW.a1; DELETE FROM test3 WHERE a3 = NEW.a1; UPDATE test4 SET b4 = b4 + 1 WHERE a4 = NEW.a1; END$$ Unlike some other DBMS products, allow procedural code in a trigger.

5 Assorted Topics Stored Procedures and Triggers Pg 5 Trigger Operation There are two important features about how a trigger executes: 1. The data in the INSERT, UPDATE, or DELETE statement that fired the trigger is made available to the trigger code. 2. The trigger syntax uses the words FOR EACH ROW. The trigger executes the code in the body once for each row affected by the INSERT, UPDATE, or DELETE statement that fired the trigger: For an INSERT, the trigger has access to the values of the rows being inserted. For a DELETE, the trigger has the values of the rows being deleted. For an UPDATE, the trigger has access to both the old and new values of the rows being changed. The nonprocedural nature of SQL is quite a work saver for the programmer. Uses for Triggers Provide calculated default values. The CREATE TABLE statement lets us define a simple default value for a column. However, if we want a calculated default value, we use an AFTER INSERT trigger. These constraints are self-contained within a single table. If we want constraints to span multiple tables, we need a trigger. Enforce data constraints. The CREATE TABLE statement contains a CHECK constraint that can: constrain a column to a range constrain a column to a set of values constrain a column against another column in the same table (for example, dateofbirth < dateofdeath) If we want to constrain a range relative to a value in a different table, we use an AFTER INSERT trigger to look up the value in the other table and act to enforce the constraint. These are the things we don t need a trigger for. We use triggers to enforce constraints across multiple tables. Update a view s underlying tables. Some views can automatically update their underlying tables, and others can t. For those that can t, we can use a trigger to update the underlying tables. Enforce minimum cardinality constraints. The Optional Child with Mandatory Parent minimum cardinality constraint can be automatically enforced by using a foreign key constraint in the table s CREATE statement. Any other minimum cardinality must rely on triggers for enforcement.

COMP 430 Intro. to Database Systems. Encapsulating SQL code

COMP 430 Intro. to Database Systems Encapsulating SQL code Want to bundle SQL into code blocks Like in every other language Encapsulation Abstraction Code reuse Maintenance DB- or application-level? DB: