Example 1: counting null values in a column

A null value is a marker used to fill a place in a column where data is missing for any reason. The value might not be applicable, or it might be missing or unknown. When a database table is created, each column in the table either allows null values or does not allow them. The column or set of columns that define the primary key cannot allow null values. Sometimes it is useful to know how many null values there are in a particular column.

What you want to do

Suppose you are working with the Fin_code table in the Enterprise Application Sample Database. The Fin_code table has three columns:

**Table 2-2: Columns in the Fin_code table**
Column	What the column is	Allows null values?
Code	Unique financial identifier (primary key)	No
Type	Code type: expense or revenue	No
Description	Code description: the department incurring the expense or getting the revenue	Yes

You create a DataWindow object using the Code and Description columns. You want to know the number of null values in the Description column.

How to do it

In the DataWindow object, you create a computed field that uses functions to display the number of null values in the Description column.

For the sake of demonstrating the use of functions, the following computed fields are created in the Summary band of the DataWindow object (with text objects that tell you what information each computed field is providing):

Count(description for all)

counts the number of descriptions (that are not null);

Sum(If(IsNull(description), 1, 0))

returns a 1 if the description column is null, a 0 if the description column is not null, and then adds the total;

Count(id for all)

counts the number of IDs (which is also the number of rows);

Sum(If(IsNull(description), 1, 1))

adds the number of nulls and not nulls in the description column (which is the total number of rows) and should match the result of the Count( id for all ) function; and

IsNull(description)

evaluates whether the last row in the table has a description that is null. The return value of the IsNull function is true or false.

What you get

Here is the design for the DataWindow object.

The sample shows the Summary band of the Data Window object. Across the top of the band the following text objects are displayed: Number of decriptions + Number of NULLs = Number of rows. Under the three text objects are the corresponding expressions Count ( description for all ) , Sum ( If ( Is Null ( description ) , 1, 0 ) ), and Count ( i d for all ). Beneath Count ( i d for all ) and connected to it by an equals sign is the expression Sum ( If ( Is Null ( description ), 1, 1) ) . At bottom left of the Summary band is the text Last value NULL? and the expression Is Null ( description ).

Here is the DataWindow object showing eight descriptions, three of which are null and five of which are not null. The last description for Id=8 is null.