外表(external table)就像普通的表对像一样，可以select等，只是它是只读的，数据库中只保存了表结构的描述，表数据却没有存放在数据库内，而是存放在了文件 系统上。当用户想偶尔使用数据库外的结构化数据时，用起外表来就非常方便，甚至比sqlldr都要方便的多。在这篇文章里，我们为大家演示了
三步就掌握oracle外表过程。通过这次学习，也许大家就会发展原来学习oracle也是好容易哦。
??
第一步：创建目录并授权。目录是数据文件的存放目标，数据文件通常要求是文本文件。这个过程在9i以前是需要配置utl_file_dir参数的。

sys@TEST>!ls /home/oracle/temp
user.ctl  userlist.txt  user.log
  
rudolf@TEST>
sys@TEST>conn system/alibaba
Connected.
sys@TEST>
sys@TEST>CREATE DIRECTORY TEMP AS '/home/oracle/temp/';
  
Directory created.
  
sys@TEST>grant read,write on directory TEMP to rudolf;
  
Grant succeeded. 

第二步：创建外表与测试

rudolf@TEST>CREATE TABLE "USERLIST"
   2  (
   3    ID NUMBER,
   4    USERNAME VARCHAR2(30),
   5    EMAIL VARCHAR2(128)
   6  )
   7  ORGANIZATION external
   8  (
   9    TYPE oracle_loader
  10    DEFAULT DIRECTORY TEMP
  11    ACCESS PARAMETERS
  12    (
  13      RECORDS DELIMITED BY NEWLINE CHARACTERSET US7ASCII
  14      BADFILE 'TEMP':'userlist.bad'
  15      DISCARDFILE 'TEMP':'userlist.dis'
  16      LOGFILE 'TEMP':'user.log'
  17      READSIZE 1048576
  18      FIELDS TERMINATED BY "," OPTIONALLY ENCLOSED BY '"' LDRTRIM
  19      MISSING FIELD VALUES ARE NULL
  20      REJECT ROWS WITH ALL NULL FIELDS
  21      (
  22        ID CHAR(30)
  23          TERMINATED BY "," OPTIONALLY ENCLOSED BY '"',
  24        USERNAME CHAR(30)
  25          TERMINATED BY "," OPTIONALLY ENCLOSED BY '"',
  26        EMAIL CHAR(128)
  27          TERMINATED BY "," OPTIONALLY ENCLOSED BY '"'
  28      )
  29    )
  30    location
  31    (
  32      'userlist.txt'
  33    )
  34  )REJECT LIMIT UNLIMITED
  35   
rudolf@TEST>/
  
Table created.
  
rudolf@TEST>l
   1  select id,username from userlist where rownum < 10
   2*
rudolf@TEST>/
  
         ID USERNAME
---------- ------------------------------
          1 RudolfLu
          3 tomgu
          6 coug
          7 chao_ping
          8 parrotao
          9 cnoug
         10 FilsDeDragon
         11 Dragon
  
9 rows selected. 

?瞧，成功了。外表就这么简单。可是只有二步啊，第三步在哪里呢？你也许会问。还有啊，userlist.txt要固定的格式吗？create table...的语法这样的狂复杂，每一项都是什么含义呢？

?

第三步：理解外表数据结构与create table ... organization external语法。大家都用过sqlldr吧？外表的数据文件的结构呢就同sqlldr能读的数据文件结构一样了。那么语法呢？嘿嘿，别急，让我们先 来做个sqlldr的练习吧：

[oracle@rac1 temp]$ head -10 userlist.txt  
1,"RudolfLu"
3,"tomgu"
6,"coug"
7,"chao_ping"
8,"parrotao"
9,"cnoug"
10,"FilsDeDragon"
11,"Dragon"
15,"Xavier"
  
[oracle@rac1 temp]$ cat user.ctl  
LOAD
INFILE '/home/oracle/temp/userlist.txt'
badfile '/home/oracle/temp/userlist.bad'
discardfile '/home/oracle/temp/userlist.dis'
APPEND
INTO TABLE userlist
fields terminated by ',' optionally enclosed by '"'
trailing nullcols
( id  char(30),
   username char(30)
)
  
rudolf@TEST>create table userlist
   2  (id  number,
   3   username varchar2(30)  
   4  );
  
Table created.
rudolf@TEST>!
[oracle@rac1 temp]$ sqlldr rudolf/nix@test2.world control=./user.ctl external_table=GENERATE_ONLY 

?注意，我们加了一个external_table的参数。它的作用是告诉sqlldr不用真实load数据，而是生成包含external table 创建脚本的log文件。

[oracle@rac1 temp]$ ls
user.ctl  userlist.txt  user.log
[oracle@rac1 temp]$ cat user.log  
  
SQL*Loader: Release 9.2.0.4.0 - Production on Wed Dec 10 20:50:19 2003
  
Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.
  
Control File:   ./user.ctl
Data File:      /home/oracle/temp/userlist.txt
   Bad File:     /home/oracle/temp/userlist.bad
   Discard File: /home/oracle/temp/userlist.dis  
  ...
CREATE DIRECTORY statements needed for files
------------------------------------
CREATE DIRECTORY SYS_SQLLDR_XT_TMPDIR_00000 AS '/home/oracle/temp/'
  
CREATE TABLE statement for external table:
------------------------------------
CREATE TABLE "SYS_SQLLDR_X_EXT_USERLIST"  
(
   ID NUMBER,
   USERNAME VARCHAR2(30)
)
ORGANIZATION exter