Cause:
SQL> drop user data;
drop user data
*
ERROR at line 1:
ORA-01940: cannot drop a user that is currently connected
Solution:
SQL> select sid, serial# from v$session where username = 'DATA';
SID SERIAL#
---------- ----------
150 873
SQL> alter system kill session '150,873';
System altered.
SQL> drop user data;
drop user data
*
ERROR at line 1:
ORA-01922: CASCADE must be specified to drop 'DATA'
SQL> drop user data cascade;
User dropped.
SQL>