Because the trigger uses the clause, it might be executed multiple times, such as when updating or deleting multiple rows.
You might omit this clause if you just want to record the fact that the operation occurred, but not examine the data for each row.
After the trigger is created, following SQL statement fires the trigger once for each row that is updated, in each case printing the new salary, the old salary, and the difference between them: Trigger names must be unique with respect to other triggers in the same schema.
For example, a table and a trigger can have the same name (however, to avoid confusion, this is not recommended). If a triggering statement includes a column list, the trigger fires only when one of the specified columns is updated.
If a triggering statement omits a column list, the trigger fires when any column of the associated table is updated.
A column list cannot be specified for statement trigger fires again.
A trigger defined on a schema fires for each event associated with the owner of the schema (the current user).
A trigger defined on a database fires for each event associated with all users. Compound triggers make it easier to program an approach where you want the actions you implement for the various timing points to share common data. When a trigger is fired, the tables referenced in the trigger action might be currently undergoing changes by SQL statements in other users' transactions.