NULL value handling

When no data can be produced for a certain field as a result of a database query and/or a join between tables, the result is normally a NULL value.

Overview

The Qlik Sense logic treats the following as real NULL values:

NULL values returned from an ODBC connection
NULL values created as a result of a forced concatenation of tables in the data load script
NULL values created as a result of a join made in the data load script
NULL values created as a result of the generation of field value combinations to be displayed in a table

It is generally impossible to use these NULL values for associations and selections, except when the NullAsValue statement is being employed.

See: NullAsValue

Text files per definition cannot contain NULL values.

Associating/selecting NULL values from ODBC

It is possible to associate and/or select NULL values from an ODBC data source. For this purpose a script variable has been defined. The following syntax can be used:

SET NULLDISPLAY=<sym>;

The symbol <sym> will substitute all NULL values from the ODBC data source on the lowest level of data input. <sym> may be any string.

In order to reset this functionality to the default interpretation, use the following syntax:

SET NULLDISPLAY=;

The use of NULLDISPLAY only affects data from an ODBC data source.

If you wish to have the Qlik Sense logic interpret NULL values returned from an ODBC connection as an empty string, add the following to your script before any SELECT statement:

SET NULLDISPLAY=";

Here '' is actually two single quotation marks without anything in between.

See: NullDisplay

Creating NULL values from text files

It is possible to define a symbol, which when it occurs in a text file or an inline clause will be interpreted as a real NULL value. Use the following statement:

SET NULLINTERPRET=<sym>;

The symbol <sym> is to be interpreted as NULL. <sym> may be any string.

In order to reset this functionality to the default interpretation, use:

SET NULLINTERPRET=;

The use of NULLINTERPRET only affects data from text files and inline clauses.

See: NullInterpret

Propagation of NULL values in expressions

NULL values will propagate through an expression according to a few logical and quite reasonable rules.

Functions

The general rule is that functions return NULL when the parameters fall outside the range for which the function is defined.

Example:

asin(2)	returns	NULL
log(-5)	returns	NULL
round(A,0)	returns	NULL

As a result of the above follows that functions generally return NULL when any of the parameters necessary for the evaluation are NULL.

Example:

sin(NULL)	returns	NULL
chr(NULL)	returns	NULL
if(NULL, A, B)	returns	B
if(True, NULL, A)	returns	NULL
if(True, A, NULL)	returns	A

The exception to the second rule are logical functions testing for type.

Example:

isnull(NULL)	returns	True (-1)
isnum(NULL)	returns	False (0)

Arithmetic and string operators

If NULL is encountered on any side of these operators NULL is returned.

Example:

A	+	NULL	returns	NULL
A	-	NULL	returns	NULL
A	/	NULL	returns	NULL
A	*	NULL	returns	NULL
NULL	/	A	returns	NULL
0	/	NULL	returns	NULL
0	*	NULL	returns	NULL
A	&	NULL	returns	A

Relational operators

If NULL is encountered on any side of relational operators special rules apply.

Example:

NULL	(any relational operator)	NULL	returns	NULL
A	<>	NULL	returns	True (-1)
A	<	NULL	returns	False (0)
A	<=	NULL	returns	False (0)
A	=	NULL	returns	False (0)
A	>=	NULL	returns	False (0)
A	>	NULL	returns	False (0)

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