Null or MISSING Values: MISSING

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If a segment instance exists but no data has been entered into one of its fields, that field has no value. Some types of data sources represent this absence of data as a blank space ( ) or zero (0), but others explicitly indicate an absence of data with a null indicator or as a special null value. Null values (sometimes known as missing data) are significant in reporting applications, especially those that perform aggregating functions, such as averaging.

If your type of data source supports missing data, as do FOCUS data sources and most relational data sources, then you can use the optional MISSING attribute to enable null values to be entered into and read from a field. MISSING plays a role when you:


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Syntax: How to Specify a Missing Value
MISSING = {ON|OFF}

where:

ON

Distinguishes a missing value from an intentionally entered blank or zero when creating new segment instances and reporting.

OFF

Does not distinguish between missing values and blank or zero values when creating new segment instances and reporting. OFF is the default value.


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Reference: Usage Notes for MISSING

Note the following rules when using MISSING:


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Using a Missing Value

Consider the field values shown in the following four records:

 
 
1
3

If you average these values without declaring the field with the MISSING attribute, a value of zero is automatically be supplied for the two blank records. Thus, the average of these four records is (0+0+1+3)/4, or 1. If you turn MISSING to ON, the two blank records are not used in the calculation, so the average is (1+3)/2, or 2.

Missing values in a unique segment are also automatically supplied with a zero, a blank, or a missing value depending on the MISSING attribute. What distinguishes missing values in unique segments from other values is that they are not stored. You do have to supply a MISSING attribute for fields in unique segments on which you want to perform counts or averages.

The Creating Reports manual contains a more thorough discussion of using null values (sometimes called missing data) in reports. It includes alternative ways of distinguishing these values in reports, such as using the WHERE phrase with MISSING selection operators, and creating virtual fields using the DEFINE FILE command with the SOME or ALL phrase.


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