Trigger:
A trigger is a stored procedure in database which automatically invokes whenever a special event in the database occurs. For example, a trigger can be invoked when a row is inserted into a specified table or when certain table columns are being updated.

Syntax:

create trigger [trigger_name] 
    [before | after]  
    {insert | update | delete}  
    on [table_name]  
    [for each row]  
    [trigger_body]

Explanation of syntax:


 Create trigger [trigger_name]: Creates or replaces an existing trigger with the trigger_name.
    [before | after]: This specifies when the trigger will be executed.
    {insert | update | delete}: This specifies the DML operation.
    on [table_name]: This specifies the name of the table 
    associated with the trigger.
    [for each row]: This specifies a row-level trigger,
     i.e., the trigger will be executed for each row being affected.
    [trigger_body]: This provides the operation to be 
    performed as trigger is fired

BEFORE and AFTER of Trigger:

BEFORE triggers run the trigger action before the triggering statement is run. AFTER triggers run the trigger action after the triggering statement is run.