SQL Server Agent executes jobs and fires alerts. As you will see in the upcoming sections, jobs and alerts are defined separately and can be executed independently. Nevertheless, jobs and alerts may also be complementary processes, because a job can invoke an alert and vice versa.
Consider an example: A job is executed to inform the system administrator about an unexpected filling of the transaction log that exceeds a tolerable limit. When this event occurs, the associated alert is invoked and, as a reaction, the system administrator may be notified by e-mail or SMS text alert.
Another critical event is a failure in backing up the transaction log. When this happens, the associated alert may invoke a job that truncates the transaction log. This reaction will be appropriate if the reason for the backup failure is an overflow (filling up) of the transaction log. In other cases (for example, the target device for the backup copy is full), such a truncation will have no effect. This example shows the close connection that may exist between events that have similar symptoms.
SQL Server Agent allows you to automate different administrative tasks. Before you can do this, the process has to be started. To start SQL Server Agent under SQL Server Management Studio, right-click the SQL Server Agent folder (under your Database Engine instance) and choose Start.
As already stated, the invocation of an alert can also include the notification of one or more operators by e-mail using Database Mail. Database Mail is an enterprise solution for sending
e-mail messages from the Database Engine. Using Database Mail, your applications can send e-mail messages to users. The messages may contain query results, and may also include files from any resource on your network.
Source: Petkovic Dusan (2020), Microsoft SQL Server 2019: A Beginner’s Guide, Seventh Edition-McGraw-Hill Education.