The following datatype or size property of <Tablename>.<Columnname> doesn't match <Tablename>.<Columnname>. (Visual Database Tools)
When creating a foreign key relationship, the data types and other properties of selected columns must match. Select a column with the same data types and other properties as the related column. The properties that must match are Data type, Length, Precision, Scale, and Collation.
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Concepts
Error Messages in Visual Database Tools