Preserving NULL values during data loading in SQL Loader.

12 Oct 2023

sqlloader

When loading data into a database using SQL Loader, it is essential to preserve the NULL values present in the input file. By default, SQL Loader considers any missing data as an empty string and loads it as such. However, by properly configuring the control file, we can preserve the NULL values during the data loading process.

Understanding NULL values

In a database, NULL represents the absence of a value or an unknown value. It differs from an empty string or zero, as it signifies missing or not applicable data. When loading data into a table, it is crucial to correctly distinguish between empty strings and NULL values.

Configuring the control file

The control file is a text file that specifies how data should be loaded into the database using SQL Loader. To preserve NULL values during the data loading process, we can make use of the NULLIF clause in the control file.

The NULLIF clause allows us to specify a condition, and if the data meets the condition, it is loaded as NULL. Here is an example of how to use the NULLIF clause in the control file:

LOAD DATA
INFILE 'datafile.txt'
INTO TABLE my_table
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
(
  column1,
  column2,
  column3 NULLIF (column3=BLANKS),
  column4
)

In the above example, the NULLIF clause is used for column3. It checks if the value is equal to BLANKS, which represents an empty string. If the condition is satisfied, the value is loaded as NULL.

Handling different conditions

Depending on the data and requirements, there may be different conditions to consider when preserving NULL values. Here are a few examples:

NULLIF with specific values:

column3 NULLIF (column3='NA')

This example sets the value of column3 as NULL if it is equal to the string ‘NA’.

NULLIF with numeric values:

column4 NULLIF (column4=0)

In this case, the value of column4 is set to NULL if it is equal to zero.

Handling different data types:

column5 NULLIF (column5=BLANKS) "TO_DATE(:column5, 'DD-MON-YYYY')"

Here, we convert the value of column5 to a date format using the TO_DATE function while setting it to NULL if it’s empty.

Conclusion

Preserving NULL values during data loading is essential for maintaining data integrity and accuracy. By configuring the control file and using the NULLIF clause appropriately, we can ensure that NULL values are correctly preserved during the SQL Loader data loading process.

#sql #sqlloader