可以执行以下语句：
select username,serial#, sid from v$session; ---查询用户会话
alter system kill session 'serial#, sid ';---删除相关用户会话

建议以后台登陆删除用户会话
1、查询oracle的连接数
select count(*) from v$session;
2、查询oracle的并发连接数
select count(*) from v$session where status='ACTIVE';
3、查看不同用户的连接数
select username,count(username) from v$session where username is not null group by username;
4、查看所有用户：
select * from all_users;
5、查看用户或角色系统权限(直接赋值给用户或角色的系统权限)：
select * from dba_sys_privs;
select * from user_sys_privs;
6、查看角色(只能查看登陆用户拥有的角色)所包含的权限
select * from role_sys_privs;
7、查看用户对象权限：
select * from dba_tab_privs;
select * from all_tab_privs;
select * from user_tab_privs;
8、查看所有角色：
select * from dba_roles;
9、查看用户或角色所拥有的角色：
select * from dba_role_privs;
select * from user_role_privs;
10、查看哪些用户有sysdba或sysoper系统权限(查询时需要相应权限)
select * from V$PWFILE_USERS;

修改数据库允许的最大连接数：
alter system set processes = 300 scope = spfile;

查看游标数量
Select * from v$open_cursor Where user_name=''

查询数据库允许的最大连接数：
select value from v$parameter where name = 'processes';
或者：show parameter processes;

查询数据库允许的最大游标数：
select value from v$parameter where name = 'open_cursors'

查看oracle版本
select banner from sys.v_$version;

按降序显示用户"SYSTEM"为每个会话打开的游标数
select o.sid, osuser, machine, count(*) num_curs from v$open_cursor o, v$session s where user_name = 'SYSTEM' and o.sid=s.sid group by o.sid, osuser, machine order by num_curs desc;

以上转载自 https://www.cnblogs.com/caogang/p/4546072.html

保留两位小数以GB为单位显示数据文件大小
SELECT ROUND(SUM(BYTES)/1024/1024/1024,2)||'GB' FROM DBA_DATA_FILES;

以MB为单位表空间分组显示数据文件大小
SELECT TABLESPACE_NAME,SUM(BYTES)/1024/1024 AS MB FROM DBA_DATA_FILES
GROUP BY TABLESPACE_NAME;

显示数据库中临时文件的大小
SELECT SUM(BYTES)/1024/1024/1024 AS GB FROM DBA_TEMP_FILES;

显示数据库中日志文件的大小
SELECT SUM(BYTES)/1024/1024/1024 AS GB FROM V$LOG;

显示数据库中数据文件、临时文件、日志文件的总大小
SELECT SUM(GB) AS GB
FROM(
SELECT SUM(BYTES)/1024/1024/1024 AS GB
FROM DBA_DATA_FILES
UNION ALL
SELECT SUM(BYTES)/1024/1024/1024
FROM DBA_TEMP_FILES
UNION ALL
SELECT SUM(BYTES)/1024/1024/1024
FROM V$LOG
);

SELECT ROUND(SUM(BYTES) / 1024 / 1024, 2) "USED(M)"
FROM DBA_SEGMENTS
GROUP BY OWNER, TABLESPACE_NAME
ORDER BY SUM(BYTES) DESC;

--1、查看所有表空间及表空间大小：
select tablespace_name ,sum(bytes) / 1024 / 1024 as MB　from dba_data_files group by tablespace_name;

--2、查看所有表空间对应的数据文件：
select tablespace_name,file_name from dba_data_files;

--3、修改数据文件大小：
alter database datafile 'H:\ORACLE\PRODUCT\10.1.0\ORADATA\ORACLE\USERS01.DBF' RESIZE 10240M;

select file_name,bytes/1024/1024 MB from dba_data_files;