SQL LOADER utility is used to load data from other data source into Oracle. For example, if you have a table in FOXPRO, ACCESS or SYBASE or any other third party database, you can use SQL Loader to load the data into Oracle Tables. SQL Loader will only read the data from Flat files. So If you want to load the data from Foxpro or any other database, you have to first convert that data into Delimited Format flat file or Fixed length format flat file, and then use SQL loader to load the data into Oracle.

Following is procedure to load the data from Third Party Database into Oracle using SQL Loader.

1. Convert the Data into Flat file using third party database command.
2. Create the Table Structure in Oracle Database using appropriate datatypes
3. Write a Control File, describing how to interpret the flat file and options to load the data.
4. Execute SQL Loader utility specifying the control file in the command line argument

To understand it better let us see the following case study.

CASE STUDY (Loading Data from MS-ACCESS to Oracle)

Suppose you have a table in MS-ACCESS by name EMP, running under Windows O/S, with the following structure

EMPNO	INTEGER
NAME	TEXT(50)
SAL	CURRENCY
JDATE	DATE

This table contains some 10,000 rows. Now you want to load the data from this table into an Oracle Table. Oracle Database is running in LINUX O/S.

Solution

Step 1

Start MS-Access and convert the table into comma delimited flat (popularly known as csv) , by clicking on File/Save As menu. Let the delimited file name be emp.csv

Now transfer this file to Linux Server using FTP command

1. Go to Command Prompt in windows
2. At the command prompt type FTP followed by IP address of the server running Oracle.
   
   FTP will then prompt you for username and password to connect to the Linux Server. Supply a valid username and password of Oracle User in Linux
   
   For example:-
   

   C:\> ftp 200.200.100.111 
   Name: oracle
   
   Password: oracle
   FTP>

3. Now give PUT command to transfer file from current Windows machine to Linux machine
   

   FTP>put
   Local file:C:\>emp.csv
   remote-file:/u01/oracle/emp.csv
   
   File transferred in 0.29 Seconds
   FTP>

4. Now after the file is transferred quit the FTP utility by typing bye command.

   FTP>bye
   Good-Bye

Step 2

Now come to the Linux Machine and create a table in Oracle with the same structure as in MS-ACCESS by taking appropriate datatypes. For example,  create a table like this

$ sqlplus scott/tiger
SQL> CREATE TABLE emp (empno number(5),
	name varchar2(50),
	sal  number(10,2),
	jdate date);

Step 3

After creating the table, you have to write a control file describing the actions which SQL Loader should do. You can use any text editor to write the control file. Now let us write a controlfile for our case study

$ vi emp.ctl

1	LOAD DATA
2	INFILE	‘/u01/oracle/emp.csv’
3	BADFILE	‘/u01/oracle/emp.bad’
4	DISCARDFILE	‘/u01/oracle/emp.dsc’
5	INSERT INTO TABLE emp 
6	FIELDS TERMINATED BY “,” OPTIONALLY ENCLOSED BY ‘”’ TRAILING NULLCOLS
7	(empno,name,sal,jdate date ‘mm/dd/yyyy’)

Notes: (Do not write the line numbers, they are meant for explanation purpose)

1.       The LOAD DATA statement is required at the beginning of the control file.
2.       The INFILE option specifies where the input file is located
3.       Specifying BADFILE is optional. If you specify,  then bad records found during loading will be stored in this file.
4.       Specifying DISCARDFILE is optional. If you specify, then records which do not meet a WHEN condition will be written to this file.
5.       You can use any of the following loading option
    i.       INSERT : Loads rows only if the target table is empty
    ii.       APPEND: Load rows if the target table is empty or not.
    iii.      REPLACE: First deletes all the rows in the existing table and then, load rows.
    iv.      TRUNCATE: First truncates the table and then load rows.
6.       This line indicates how the fields are separated in input file. Since in our case the fields are separated by “,” so we have specified “,” as the terminating char for fields. You can replace this by any char which is used to terminate fields. Some of the popularly use terminating characters are semicolon “;”, colon “:”, pipe “|” etc. TRAILING NULLCOLS means if the last column is null then treat this as null value, otherwise,  SQL LOADER will treat the record as bad if the last column is null.
7.        In this line specify the columns of the target table. Note how do you specify format for Date columns

Step 4

After you have wrote the control file save it and then, call SQL Loader utility by typing the following command

$sqlldr userid=scott/tiger control=emp.ctl log=emp.log 

After you have executed the above command SQL Loader will shows you the output describing how many rows it has loaded.

The LOG option of sqlldr specifies where the log file of this sql loader session should be created. The log file contains all actions which SQL loader has performed i.e. how many rows were loaded, how many were rejected and how much time is taken to load the rows and etc. You have to view this file for any errors encountered while running SQL Loader.

Conventional Path Load and Direct Path Load.

SQL Loader can load the data into Oracle database using Conventional Path method or Direct Path method. You can specify the method by using DIRECT command line option. If you give DIRECT=TRUE then SQL loader will use Direct Path Loading otherwise, if omit this option or specify DIRECT=false, then SQL Loader will use Conventional Path loading method.

Conventional Path

Conventional path load (the default) uses the SQL INSERT statement and a bind array buffer to load data into database tables.

When SQL*Loader performs a conventional path load, it competes equally with all other processes for buffer resources. This can slow the load significantly. Extra overhead is added as SQL statements are generated, passed to Oracle, and executed.

The Oracle database looks for partially filled blocks and attempts to fill them on each insert. Although appropriate during normal use, this can slow bulk loads dramatically.

Direct Path

In Direct Path Loading, Oracle will not use SQL INSERT statement for loading rows. Instead it directly writes the rows, into fresh blocks beyond High Water Mark, in datafiles i.e. it does not scan for free blocks before high water mark. Direct Path load is very fast because

• Partial blocks are not used, so no reads are needed to find them, and fewer writes are performed.
• SQL*Loader need not execute any SQL INSERT statements; therefore, the processing load on the Oracle database is reduced.
• A direct path load calls on Oracle to lock tables and indexes at the start of the load and releases them when the load is finished. A conventional path load calls Oracle once for each array of rows to process a SQL INSERT statement.
• A direct path load uses multiblock asynchronous I/O for writes to the database files.
• During a direct path load, processes perform their own write I/O, instead of using Oracle's buffer cache. This minimizes contention with other Oracle users.

Restrictions on Using Direct Path Loads

The following conditions must be satisfied for you to use the direct path load method:

• Tables are not clustered.
• Tables to be loaded do not have any active transactions pending.
• Loading a parent table together with a child Table
• Loading BFILE columns

There are also third party tools available which makes migration very easy. Most of these tools are commercial but they make conversion job very easy. One such tool you use is Data Loader and you can download the free trial edition here https://www.dbload.com

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