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If your database column allows NULL then the XSD schema generated by the frevvo database connector will set minOccurs='0' for this control. When this control is used in your form the field will not be required. To make this field required you have to edit the XSD to set minOccurs='1' which makes the XSD schema more restrictive then your database schema.
See also Data Sources - Required Schema Controls.
Changing text controls to date Controls
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