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。

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代码:

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

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代码:

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 **

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？

这就是第三步要教给大家的东西了。

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,"**"

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 external

(

TYPE oracle_loader

DEFAULT DIRECTORY SYS_SQLLDR_XT_TMPDIR_00000

ACCESS PARAMETERS

(

RECORDS DELIMITED BY NEWLINE CHARACTERSET US7ASCII

BADFILE 'SYS_SQLLDR_XT_TMPDIR_00000':'userlist.bad'

DISCARDFILE 'SYS_SQLLDR_XT_TMPDIR_00000':'userlist.dis'

LOGFILE 'user.log_xt'

READSIZE 1048576

FIELDS TERMINATED BY "," OPTIONALLY ENCLOSED BY '"' LDRTRIM

MISSING FIELD VALUES ARE NULL

REJECT ROWS WITH ALL NULL FIELDS

(

ID CHAR(30)

TERMINATED BY "," OPTIONALLY ENCLOSED BY '"',

USERNAME CHAR(30)

TERMINATED BY "," OPTIONALLY ENCLOSED BY '"' )

)

location

(

'userlist.txt'

)

)REJECT LIMIT UNLIMITED

...

瞧，原来我们更本不用担心怎么写create external table的语句呢。sqlldr就可以帮我们生成了！