External Table外部表

1. 概念
    外部表是存储于数据库外的一种表,它允许以只读的形式读取外部表里的数据. 你可以像普通表一样对外部表进行SELECT, SORT, JOIN操作,也可以在外部表上建视图,同义词等,但不能有DML操作.

   外部表的定义也就像创建一个视图一样,查询外部表的数据不用先把数据装载进外部表里.对外部表的访问主要是通过ACCESS DRIVER来实现的.

Access Driver:
Oracle 提供两种访问驱动: ORACLE_LOADER, ORACLE_DATAPUMP
ORACLE_LOADER: 它利用oracle loader技术从外部表读取数据. 它还具有类似SQL*Loader工具控制文件语法的数据映射能力.

ORACLE_DATAPUMP: 它提供卸载数据的能力,即把数据从数据库里导入一个外部表,再由一个或几个外部表导入到数据库里. 对于AS subquery的子句必须使用ORACLE_DATAPUMP.

创建外部表实例:

The file empxt1.dat contains the following sample data:

360,Jane,Janus,ST_CLERK,121,17-MAY-2001,3000,0,50,jjanus
361,Mark,Jasper,SA_REP,145,17-MAY-2001,8000,.1,80,mjasper
362,Brenda,Starr,AD_ASST,200,17-MAY-2001,5500,0,10,bstarr
363,Alex,Alda,AC_MGR,145,17-MAY-2001,9000,.15,80,aalda

The file empxt2.dat contains the following sample data:

401,Jesse,Cromwell,HR_REP,203,17-MAY-2001,7000,0,40,jcromwel
402,Abby,Applegate,IT_PROG,103,17-MAY-2001,9000,.2,60,aapplega
403,Carol,Cousins,AD_VP,100,17-MAY-2001,27000,.3,90,ccousins
404,John,Richardson,AC_ACCOUNT,205,17-MAY-2001,5000,0,110,jrichard

The following hypothetical SQL statements create an external table in the hr schema named admin_ext_employees and load its data into the hr.employees table.

CONNECT  /  AS SYSDBA;
-- Set up directories and grant access to hr 
CREATE OR REPLACE DIRECTORY admin_dat_dir
    AS '/flatfiles/data'; 
CREATE OR REPLACE DIRECTORY admin_log_dir 
    AS '/flatfiles/log'; 
CREATE OR REPLACE DIRECTORY admin_bad_dir 
    AS '/flatfiles/bad'; 
GRANT READ ON DIRECTORY admin_dat_dir TO hr; 
GRANT WRITE ON DIRECTORY admin_log_dir TO hr; 
GRANT WRITE ON DIRECTORY admin_bad_dir TO hr;
-- hr connects. Provide the user password (hr) when prompted.
CONNECT hr
-- create the external table
CREATE TABLE admin_ext_employees
                   (employee_id       NUMBER(4), 
                    first_name        VARCHAR2(20),
                    last_name         VARCHAR2(25), 
                    job_id            VARCHAR2(10),
                    manager_id        NUMBER(4),
                    hire_date         DATE,
                    salary            NUMBER(8,2),
                    commission_pct    NUMBER(2,2),
                    department_id     NUMBER(4),
                    email             VARCHAR2(25) 
                   ) 
     ORGANIZATION EXTERNAL 
     ( 
       TYPE ORACLE_LOADER 
       DEFAULT DIRECTORY admin_dat_dir 
       ACCESS PARAMETERS 
       ( 
         records delimited by newline 
         badfile admin_bad_dir:'empxt%a_%p.bad' 
         logfile admin_log_dir:'empxt%a_%p.log' 
         fields terminated by ',' 
         missing field values are null 
         ( employee_id, first_name, last_name, job_id, manager_id, 
           hire_date char date_format date mask "dd-mon-yyyy", 
           salary, commission_pct, department_id, email 
         ) 
       ) 
       LOCATION ('empxt1.dat', 'empxt2.dat') 
     ) 
     PARALLEL 
     REJECT LIMIT UNLIMITED; 
-- enable parallel for loading (good if lots of data to load)
ALTER SESSION ENABLE PARALLEL DML;
-- load the data in hr employees table
INSERT INTO employees (employee_id, first_name, last_name, job_id, manager_id,
                       hire_date, salary, commission_pct, department_id, email) 
            SELECT * FROM admin_ext_employees;

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