What is the difference between execution of triggers and stored procedures?

In PL/SQL, triggers and stored procedures serve different purposes, and there are several key differences between their execution:

1. Trigger Execution:
   • Triggers are automatically executed or fired in response to specific events on a particular table or view. These events include INSERT, UPDATE, DELETE, or even DDL (Data Definition Language) statements like CREATE, ALTER, or DROP.
   • Triggers are implicitly invoked and do not require direct user intervention. They are associated with a specific table or view and are executed automatically when the triggering event occurs.
2. Stored Procedure Execution:
   • Stored procedures, on the other hand, are explicitly called by a user or another program. They are not triggered by events on a table.
   • Stored procedures are standalone units of code that perform a specific task or set of tasks. They need to be explicitly invoked by a user or an application using a CALL statement or as part of a SQL block.
3. Event Context:
   • Triggers are closely tied to the events occurring on a table or view, and they have access to special variables like NEW and OLD that represent the new and old values of the data being modified.
   • Stored procedures do not have access to these special variables, as they are not inherently linked to specific database events.
4. Timing of Execution:
   • Triggers can be classified based on the timing of their execution into BEFORE triggers, AFTER triggers, and INSTEAD OF triggers. These define whether the trigger logic is executed before or after the triggering event.
   • Stored procedures are executed explicitly when they are called and do not have the concept of timing in the same way as triggers.
5. Transaction Control:
   • Triggers can be used to enforce complex integrity constraints and can implicitly participate in the transaction control mechanism, affecting the outcome of the triggering statement.
   • Stored procedures can also participate in transaction control, but this is typically done explicitly through the use of COMMIT and ROLLBACK statements within the procedure.

In summary, triggers are automatically invoked in response to specific database events, while stored procedures are explicitly called by users or applications. Triggers are closely tied to the database events they are associated with, whereas stored procedures are standalone units of code with no inherent connection to database events.