Oracle常用的SQL方法总结

在项目中一般需要对一些数据进行处理,以下提供一些基本的SQL语句: 

1.基于条件的插入和修改:需要在表中插入一条记录,插入前根据key标识判断。如果标识符不存在,则插入新纪录,如果标识符存在,则根据语句中所给的新值对原纪录中的字段进行更新:

merge into A
using B
on (A.key = B.key)
when matched then
  update set A.name = B.name
when not matched then
  insert into (A.key, A.name) values (B.key, B.name)

2.在分组和集合里统计分组:当需要统计一个分组里的成员,或有多少个子分组,及其他基于集合的统计。在进行基于集合的统计时,还需要动态包括或剔除满足特定条件的记录。

实例如下:基于职工在企业里的升职次数,来统计企业每个员工经历过的工作岗位次数。

select jh.JobsHeld, count(*) as StaffCount
      from (select u.employee_id, count(*) as JobsHeld
              from (select employee_id
                      from hr.employees
                    union all
                    select employee_id from hr.job_history) u
            group by u.employee_id) jh
    group by jh.JogsHeld

 3.根据时间戳恢复数据:

ALTER TABLE tablename ENABLE row movement ;
flashback table tablename to timestamp to_timestamp('2012-09-13 13:00:00','yyyy-mm-dd hh24:mi:ss');

4.有一个类型为字符串的字段,要讲其值迁移到新系统里的数值类型字段中:

create or replace function isnum(v_in varchar2) return varchar is
  val_err exception;
  pragma exception_init(val_err, -6502) --char to num conv.error
  scrub_num number;
begin
  scrub_num := to_number(v_in) return 'Y';
exception
  when val_err then
    return 'N';
end;

5.解决死锁问题:

(1).查看被锁的表
  select b.owner,b.object_name,a.session_id,a.locked_mode
from v$locked_object a,dba_objects b
where b.object_id = a.object_id;
 
  (2).查看引起死锁的会话
  select b.username,b.sid,b.serial#,logon_time
from v$locked_object a,v$session b
where a.session_id = b.sid order by b.logon_time;

  (3).杀掉死锁
alter system kill session ‘sid,serial#’;

6.查看当前连接用户正在运行的SQL语句:

select a.SID, a.USERNAME, b.SQL_TEXT
  from v$session a, v$sqltext_with_newlines b
 where a.SQL_ADDRESS = b.ADDRESS
  and a.SQL_HASH_VALUE = b.HASH_VALUE
 order by a.USERNAME, a.SID, b.piece

7.查询最消耗资源的实时语句:

Oracle 11g中,可以通过查询V$SQL_MONITOR来监控SQL查询近乎实时的消耗资源
  select * from (
    select a.sid session_id,a.sql_id,a.status,a.cpu_time/1000000 cpu_sec,a.buffer_gets,a.disk_reads,b.sql_text sql_text
    from v$sql_monitor a,v$sql_b
    where a.sql_id=b.sql_id
    order by a.cpu_time desc)
    where rownum<=20;

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