66. Triggers in SQL|DDL Triggers|DML Triggers|Magic tables in SQL|SQL Training by Pandey Guruji

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The trigger is a piece of procedural code, like a stored procedure that is only executed
when a given event happens. There are different types of events that can fire a trigger.
Just to name a few, the insertion of rows in a table, a change in a table structure, and
even a user logging into a SQL Server instance.
There are three main characteristics that make triggers different than stored
procedures:
● Triggers cannot be manually executed by the user.
● There is no chance for triggers to receive parameters.
● You cannot commit or roll back a transaction inside a trigger.
The fact that it's impossible to use parameters on triggers is not a limitation to receiving
information from the firing event. As you will see further on, there are alternatives to
obtain information about the firing event.
There are two classes of triggers in SQL Server:
● DDL (Data Definition Language) triggers. This class of triggers fires upon events
that change the structure (like creating, modifying, or dropping a table), or in
certain server-related events like security changes or statistics update events.
● DML (Data Modification Language) triggers. This is the most used class of
triggers. In this case, the firing event is a data modification statement; it could be
an insert, update or delete statement either on a table or a view.
Additionally, DML triggers have different types:
● FOR or AFTER [INSERT, UPDATE, DELETE]: These types of triggers are executed
after the firing statement ends (either an insert, update or delete).
● INSTEAD OF [INSERT, UPDATE, DELETE]: Contrary to the FOR (AFTER) type, the
INSTEAD OF triggers executes instead of the firing statement. In other words,
this type of trigger replaces the firing statement. This is very useful in cases
where you need to have cross-database referential integrity.
What is The Importance of SQL Server Triggers?
One of the fundamental characteristics of relational databases is data consistency. This
means that the information stored in the database must be consistent at all times for
every session and every transaction. The way relational database engines like SQL
Server implement this is by enforcing constraints like primary keys and foreign keys. But
sometimes that is not enough.

Magic Table in SQL- Inserted & Deleted.

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