Updating tables through object views in oracle
If the column list after the INSERT keyword is left out, the number of columns in the target table must match the number of values in the VALUES clause.
The WHERE condition can apply to either the data source or the target table.
The MERGE statement is designed to combine multiple operations to reduce the complexity of mixed insert and update operations.
MERGE allows you to avoid multiple INSERT, UPDATE, and DELETE DML statements by combining the operations into a single statement.
Even if a row of the target table satisfies the DELETE condition but is not included in the data set from the join defined by the MERGE's ON clause, then it is not deleted.
If the MERGE statement deletes a row, any delete triggers defined on the target table will be activated for each row deletion. The WHERE clause can be specified by itself or in concert with the merge_insert_clause.