Before you can use QlikView Expressor to read from or write to an external data source, you must prepare a Schema artifact. The Schema artifact describes the structure of the external data including the field name, value and data type. Schemas describing data in a delimited file assign the string type to all fields and attributes whereas Schemas describing database tables derive each field's type from the table's metadata and assign a comparable type to the corresponding attribute.
With the delimited or table Schema, if you want to change the type of any field as it is represented in the composite type, you open the Schema in the schema editor, change the type of the attribute and, if necessary, use the schema mapping to describe to Expressor how to convert the value from its original type to the desired type (see http://community.qlik.com/docs/DOC-3232 for background information).
Schemas that describe Excel worksheets are a little different. Like a table Schema, the type of each field, corresponding to a column in the worksheet, is derived from the worksheet's metadata and the corresponding attribute in the composite type has a comparable type. When you open an Excel Schema in the schema editor, you will see that the mappings cannot be edited. Therefore, if you want to change the type of a field, you must modify the Schema, changing the type assigned to both the field and its corresponding attribute.
The following video illustrates how you use the Schema Editor to perform these modifications.