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The Propagation of Nulls and Query Results
Null values can wreak havoc with your query results because they propagate. Take a look at the query in Figure 11.35. Notice that when parts and labor are added, and either the Parts field or the Labor field contains a Null, the result of adding the two fields is Null. In Figure 11.36, the problem is rectified. Figure 11.37 shows the design of the query that eliminates the propagation of the Nulls. Notice the expression that adds the two values:TotalPrice: Nz([Parts]) + Nz([Labor])
Figure 11.35. An example that shows the propagation of Nulls in a query result.
Figure 11.36. An example that shows Nulls eliminated from the query result.
Figure 11.37. A solution to eliminate propagation of Nulls.
This expression uses the Nz function to convert the Null values to 0 before the two field values are added together.
