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The exceptions (or restrictions) include views that use aggregate functions; group functions; use of the DISTINCT keyword; use of GROUP BY, CONNECT BY or START WITH clauses; and use of some joins.
In many cases, use of the INSTEAD-OF trigger feature allows you to work around these restrictions.
It says that if the trigger reads the table (such as using a SELECT query), that changes (even using : NEW) will fail.
This can also happen when a trigger on a parent table causes an insert on a child table referencing a foreign key.
In many cases, the cause of this error is due to code within a trigger that looks at or touches the data within the table the trigger is being called or invoked from.
The reason Oracle raises this error is related to one of Oracle's primary strengths as a relational database management system.
There are exceptions to this rule about views being inherently updateable.This data can be stored in a PL/SQL collection or in a temporary table.A simple row level trigger that causes a mutating table error can result in a very complicated statement level trigger to achieve the needed result.Perhaps the greatest strength or utility of an INSTEAD-OF trigger is its ability to update what would normally appear to be non-updateable views.Simple views (pretty much based on a single base table) generally are inherently updateable via DML statements issued against the view.