Extracting and appending computed fields

You cannot append computed and physical fields to each other. If the source and/or target table contains computed fields, the potential exists for a mismatch between corresponding computed and physical fields. The mismatch results in jumbled data if you force the append operation (by selecting Append to Existing File in the Extract dialog box).

The option you select when extracting data – Record, View, or Fields – can either create or reconcile a mismatch. Selecting Record preserves extracted computed fields as computed expressions. Selecting View or Fields converts extracted computed fields to physical fields and populates them with the actual computed values. You need to select the appropriate option, and in some cases perform preparatory steps (outlined below), when extracting and appending involves computed fields.

Table 1 summarizes the different possible field combinations and outlines the steps necessary to reconcile computed and physical fields.

Two corresponding computed fields may also use expressions that differ. In this situation, the target expression takes precedence over the source expression, which may also require that you perform some preparatory steps to ensure the combined data is valid.

Table 1. Reconciling computed and physical fields

Target: computed field

Target: physical field

When using the Extract Record option

Source: computed field

No mismatch when appending.

If source and target computed fields use different expressions, the target field expression takes precedence, which may make computed values in the appended source field invalid. Review both expressions to ensure the difference is warranted. If it is, you can use the extract View or extract Fields option to convert both the source and target computed fields to physical fields in new tables prior to appending the new tables.

Mismatch when appending.

Remedy: Use the extract View or extract Fields option to extract data from the source table.

Source: physical field

Mismatch when appending.

Remedy: Use the extract View or extract Fields option to create a new target table from the existing target table. Extract the required records or fields from the source table and append them to the new target table.

No mismatch when appending.

When using the Extract View or Extract Fields option

Source: computed field

Mismatch when appending.

Remedy: Use the extract Record option to extract data from the source table.

No mismatch when appending.

Source: physical field

Mismatch when appending.

Remedy: Use the extract View or extract Fields option to create a new target table from the existing target table. Extract the required records or fields from the source table and append them to the new target table.

No mismatch when appending.



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