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Saturday, 2 April 2016

SQL External Tables

You can use external table feature to access external files as if they are tables inside the database.
When you create an external table, you define its structure and location with in oracle.
When you query the table, oracle reads the external table and returns the results just as if the data had been stored with in the database.

ACCESSING EXTERNAL TABLE DATA

To access external files from within oracle, you must first use the create directory command to define a directory object pointing to the external file location
Users who will access the external files must have the read and write privilege on the directory.

Ex:

CREATING DIRECTORY AND OS LEVEL FILE

    SQL> Sqlplus system/manager
    SQL> Create directory saketh_dir as ‘/Visdb/visdb/9.2.0/external’;
     SQL> Grant all on directory saketh_dir to saketh;
     SQL> Conn saketh/saketh
     SQL> Spool dept.lst
     SQL> Select deptno || ‘,’ || dname || ‘,’ || loc from dept;
     SQL> Spool off

CREATING EXTERNAL TABLE

 SQL> Create table dept_ext
         (deptno number(2),
         Dname varchar(14),
         Loc varchar(13))
         Organization external  ( type oracle_loader
                                                 Default directory saketh_dir
                                                 Access parameters
                                                 ( records delimited by newline
                                                    Fields terminated by “,”
                                                    ( deptno number(2),
                                                      Dname varchar(14),
                                                      Loc varchar(13)))
         Location (‘/Visdb/visdb/9.2.0/dept.lst’));

SELECTING DATA FROM EXTERNAL TABLE

SQL> select * from dept_ext;
This will read from dept.lst which is a operating system level file.

LIMITATIONS ON EXTERNAL TABLES

a)     You can not perform insert, update, and delete operations
a)     Indexing not possible
b)     Constraints not possible

BENEFITS OF EXTERNAL TABLES

a)   Queries of external tables complete very quickly even though a full table scan id required with each access


b)   You can join external tables to each other or to standard tables
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