An expression field defines the calculations to perform on an incoming field and acts as the output field for the results. You can use as many expression fields as necessary to perform calculations on incoming fields.
When you configure an expression field, you define different fields based on the field type, for example the field name, data type, precision, scale, default value, and optional description. The description can contain up to 4000 characters. You also define the calculations that you want to perform.
You cannot specify an expression decimal field precision value greater than 38.
Effect of the default value
The default value tells the mapping task what to do when the transformation encounters output errors. The default value is not available in mappings in advanced mode.
You can set one of the following values for output fields:
ERROR('transformation error'). System default. When a transformation error occurs, the mapping task skips the row and writes the error to the session log or row error log.
A constant or constant expression. The mapping task replaces the error with the constant or constant expression. Nothing is written to the logs.
ABORT. Transformation aborts and the mapping task writes a message to the session log.
Data Integration validates the output field default value when you save or validate the mapping. If you enter an invalid default value, the Mapping Designer marks the mapping as not valid.