SQL>
SQL> UNDEFINE user_name
SQL> VARIABLE l_user VARCHAR2(30)
SQL> EXECUTE :l_user := '&user_name';
Enter value for user_name:
PL/SQL procedure successfully completed.
SQL> SELECT 'User ' || :l_user || ' has ' || TO_CHAR(COUNT(*)) || ' tables.'
2 FROM all_tables
3 WHERE owner = UPPER(:l_user);
User has 0 tables.
1 row selected.
SQL>