Database triggers are special automated procedures in a database that are automatically executed when certain events occur on a table or view.
Imagine you have a table called Orders, and you want to automatically log every time an order is deleted.
You can create a DELETE trigger on the Orders table that inserts a message into a Log table whenever a row is deleted.
| Type | Description |
|---|---|
| BEFORE | Executes before the triggering action |
| AFTER | Executes after the triggering action |
| INSTEAD OF | (for views) replaces the triggering action |
CREATE TRIGGER log_delete
AFTER DELETE ON Orders
FOR EACH ROW
BEGIN
INSERT INTO Log (action, timestamp)
VALUES ('Order deleted', NOW());
END;Data validation
Audit logging
Enforcing business rules
Extending referential integrity
Can be hard to debug
Might trigger other actions unexpectedly
Can impact performance if overly complex
Data Manipulation Language (DML) is a subset of SQL (Structured Query Language) used to manage and manipulate data within a database. With DML, users can insert, query, update, and delete data — essentially everything you'd typically do with data stored in a database.
| Command | Purpose |
|---|---|
SELECT |
Retrieve data from a table |
INSERT |
Add new data |
UPDATE |
Modify existing data |
DELETE |
Remove data |
-- Insert
INSERT INTO customers (name, city) VALUES ('Müller', 'Berlin');
-- Query
SELECT * FROM customers WHERE city = 'Berlin';
-- Update
UPDATE customers SET city = 'Hamburg' WHERE name = 'Müller';
-- Delete
DELETE FROM customers WHERE name = 'Müller';DML deals with the data inside tables, not with the structure of the tables themselves (that's handled by Data Definition Language, DDL).
DML operations can often be rolled back (undone), especially when transactions are supported.
In short: DML is the toolset you use to keep your database dynamic and interactive by constantly adding, reading, modifying, or deleting data.
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