1. 查询第一条记录

        where语句后面跟上  and rownum=1 

     2.日期所相差分钟数

       ceil((LOGOUT_TIME - LOGIN_TIME) * 24 * 60) 

     3.group by分组后，取得分组前的每一组的第一条记录

       ---获取15届学生第一次登录wifi的最早时间

       select * from (

       select wifilogin.*,row_number() over(partition by USER_ID order by rownum)cn from T_SAM_YHDLXX

        ) wifilogin

         )where cn='1'

       4.查询某用户被赋予的表权限

          SELECT * FROM user_tab_privs_made where grantee='USR_GIS'

   5. 按名称查询视图

      select view_name from user_views where view_name like 'V%'

6. 查询数据表所占用的空间

select OWNER, t.segment_name, t.segment_type, sum(t.bytes / 1024 / 1024) mmm
from dba_segments t
where t.owner = '用户名'
and t.segment_type='TABLE'
--and t.segment_name='表名'
group by OWNER, t.segment_name, t.segment_type
order by mmm desc;
7，查询库中所含的表、列、数据行（

https://www.cnblogs.com/JohanChan/p/11718183.html）

SELECT COUNT(*) TABLES, table_schema FROM information_schema.TABLES   WHERE table_schema = 'szdb'

SELECT COUNT(column_name) FROM information_schema.COLUMNS WHERE TABLE_SCHEMA = 'szdb';

use information_schema;

SELECT sum(table_rows) from tables where TABLE_SCHEMA = 'szdb' order by table_rows desc;