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In the realm of computer science, SQL Triggers play a crucial role in maintaining and manipulating data within Databases. This comprehensive guide will delve into the world of SQL Triggers, exploring their importance and functionality in detail. You will gain a thorough understanding of SQL Triggers, including their basic concepts and working mechanisms, as well as the various types and use cases of triggers. Furthermore, this guide will provide practical examples to create, modify, and delete SQL Triggers. Additionally, you will learn the difference between SQL Triggers and Functions, helping you make informed decisions on when to use each. Performance comparisons and best practices will also be discussed, offering valuable insights. Finally, this guide will cover how SQL Triggers interact with Inserted Tables, while providing practical examples to demonstrate their usage in ensuring data integrity and tracking table changes. With this knowledge at your disposal, you will be well-equipped to implement SQL Triggers effectively in your projects.
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Jetzt kostenlos anmeldenIn the realm of computer science, SQL Triggers play a crucial role in maintaining and manipulating data within Databases. This comprehensive guide will delve into the world of SQL Triggers, exploring their importance and functionality in detail. You will gain a thorough understanding of SQL Triggers, including their basic concepts and working mechanisms, as well as the various types and use cases of triggers. Furthermore, this guide will provide practical examples to create, modify, and delete SQL Triggers. Additionally, you will learn the difference between SQL Triggers and Functions, helping you make informed decisions on when to use each. Performance comparisons and best practices will also be discussed, offering valuable insights. Finally, this guide will cover how SQL Triggers interact with Inserted Tables, while providing practical examples to demonstrate their usage in ensuring data integrity and tracking table changes. With this knowledge at your disposal, you will be well-equipped to implement SQL Triggers effectively in your projects.
A trigger is a named database object that performs a specified action automatically when an event occurs, such as an INSERT, UPDATE, DELETE, or TRUNCATE statement on a specified table or view. SQL triggers enable you to perform additional actions in response to changes in data, helping to maintain the integrity of your database.
Suppose you have an online store with a table named orders. Whenever a new order is INSERTED, you want to decrease the available stock of the product by the given quantity. A trigger can be created on the orders table that gets executed whenever a new order is INSERTED, and this trigger would then automatically update the stock table.
BEFORE INSERT
, BEFORE UPDATE
, BEFORE DELETE
): These triggers execute before the triggering event. They can be used to modify the data before it is saved in the database, which is useful for data validation or cleanup.AFTER INSERT
, AFTER UPDATE
, AFTER DELETE
): These triggers execute after the triggering event. They can be useful for actions that depend on the changes made by the event, such as maintaining a record of changes or updating related tables.It's important to note that TRUNCATE cannot have a trigger directly associated with it because it's not logged on a per-row basis. However, it is possible to create a trigger on an auxiliary table and then use a DELETE statement on that table to trigger the desired action.
FOR EACH ROW
): These triggers are executed once for each row affected by the triggering event. They are useful for operations that need to be applied to individual records, such as maintaining a history of changes for each row.A practical example of a row-level trigger would be keeping track of the last modification date for each row in a table. As a new row is INSERTED or an existing row is UPDATED, the trigger would update a "last_modified" column for the affected row.
CREATE TRIGGER trigger_name {BEFORE|AFTER} {INSERT|UPDATE|DELETE} ON table_name [FOR EACH ROW] trigger_body;
CREATE TRIGGER
: This is the command used to create a new trigger.trigger_name
: A unique name for the trigger.BEFORE|AFTER
: The trigger timing, determining when the trigger action is executed with respect to the triggering event.INSERT|UPDATE|DELETE
: The triggering event that activates the trigger.ON table_name
: Specifies the table on which to create the trigger.FOR EACH ROW
: Optional modifier for row-level triggers.trigger_body
: The code that will be executed when the trigger is activated.CREATE TRIGGER trg_salary_history AFTER UPDATE OF salary ON employees FOR EACH ROW INSERT INTO employees_salary_history (employee_id, old_salary, new_salary, changed_on) VALUES (:old.employee_id, :old.salary, :new.salary, CURRENT_TIMESTAMP);In this example, the trigger is named 'trg_salary_history', it is executed after an update of the 'salary' column in the 'employees' table, and it is a row-level trigger, which is specified by the 'FOR EACH ROW' clause. When the trigger executes, it inserts a new row in the 'employees_salary_history' table with the employee_id, old_salary, new_salary, and the change timestamp.
ALTER TRIGGER trg_salary_history AFTER UPDATE OF salary ON employees FOR EACH ROW INSERT INTO employees_salary_history (employee_id, old_salary, new_salary, changed_on, updated_by) VALUES (:old.employee_id, :old.salary, :new.salary, CURRENT_TIMESTAMP, USER);This code modifies the 'trg_salary_history' trigger by adding 'updated_by' to the INSERT statement in the trigger body.
DROP TRIGGER trg_salary_history;
This code removes the 'trg_salary_history' trigger from the database. Note that dropping a trigger does not affect the table or the data in the table, it only removes the trigger functionality.
SQL Triggers:
Inserted Table is a temporary, in-memory table created by the Database Management System (DBMS) whenever a trigger is executed due to an INSERT, UPDATE, or DELETE event. Inserted tables hold the new updated values for data manipulation operations and can be accessed by the trigger during its execution to reference or compare the data.
Here are some examples of SQL Triggers being used.
CREATE TRIGGER trg_salary_log AFTER UPDATE OF salary ON employees FOR EACH ROW BEGIN INSERT INTO salary_audit (employee_id, old_salary, new_salary, modified_at) VALUES (:old.employee_id, :old.salary, :new.salary, CURRENT_TIMESTAMP); END;In this example, the trigger trg_salary_log is activated after the salary column in the employees table is updated. It then inserts a new row into the salary_audit table, capturing the employee_id, old_salary, new_salary, and modification timestamp.
CREATE TRIGGER trg_validate_order BEFORE INSERT ON orders FOR EACH ROW BEGIN DECLARE @stock INT; SELECT stock INTO @stock FROM products WHERE product_id = :new.product_id; IF @stock < :new.quantity THEN ROLLBACK; -- If the requested quantity is more than available stock, roll back the transaction RETURN; END IF; END;In this example, the trg_validate_order trigger is activated before a new row is inserted into the orders table. It checks the requested order quantity against the available stock using the inserted table values. If the requested quantity exceeds the available stock, the transaction is rolled back, ensuring data integrity in the database.
SQL Triggers: Named database objects automatically executing actions (INSERT, UPDATE, DELETE, TRUNCATE) when an event occurs on a specified table or view, maintaining data integrity within Databases.
Basic Concepts of SQL Triggers: Trigger Event (e.g., INSERT, UPDATE, DELETE), Trigger Action (modifications to tables), Triggering Event (data manipulation operations), Triggering Table (table being modified).
SQL Triggers Types: Before Triggers (before the triggering event), After Triggers (after the triggering event), Row-Level Triggers (executed for each row affected), Statement-Level Triggers (executed once per event).
Difference between SQL Triggers and Functions: Triggers are automatically executed in response to data manipulation events, while Functions are explicitly called to return values. Triggers enforce data integrity, whereas Functions perform calculations and data manipulation.
SQL Triggers Inserted: Temporary in-memory table created during trigger execution, enabling the trigger to reference and compare new values for data manipulation operations, ensuring data integrity.
Flashcards in SQL Triggers59
Start learningWhat is a SQL trigger?
A named database object that performs a specified action automatically when an event occurs (e.g., INSERT, UPDATE, DELETE, or TRUNCATE) on a specified table or view, helping maintain database integrity.
What are Before and After triggers in SQL?
Before triggers execute before the triggering event and can modify data before it is saved, while After triggers execute after the event and are useful for actions dependent on the changes made by the event.
How do Row-Level and Statement-Level triggers differ in SQL?
Row-Level triggers execute once for each row affected by the event, while Statement-Level triggers execute once for each triggering event, regardless of the number of affected rows.
Why can TRUNCATE not have a trigger directly associated with it?
TRUNCATE cannot have a trigger directly associated because it is not logged on a per-row basis. Instead, a trigger can be created on an auxiliary table and triggered with a DELETE statement.
What is the basic syntax for creating an SQL trigger?
CREATE TRIGGER trigger_name {BEFORE|AFTER} {INSERT|UPDATE|DELETE} ON table_name [FOR EACH ROW] trigger_body;
How do you alter an existing SQL trigger?
Use the 'ALTER TRIGGER' syntax followed by the trigger name, trigger timing, triggering event, table name, and the updated trigger_body.
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