You can use LONG columns in UPDATE, INSERT, and (most) SELECT statements, but not in expressions, function calls, or SQL clauses such as WHERE, GROUP BY, and CONNECT BY. Only one LONG column is allowed in each database table and that column cannot be indexed.
SQL> CREATE TABLE test
2 (a LONG);
Table created
SQL>
SQL> INSERT INTO test
2 SELECT 1 FROM dual;
1 row inserted
SQL> UPDATE test a SET a.a=2 ;
1 row updated
SQL> UPDATE test SET a=3 WHERE a=2;
UPDATE test SET a=3 WHERE a=2
ORA-00997: illegal use of LONG datatype
SQL>