When installing a module that had the following in the xmldb install.xml field definition:
<FIELD NAME="type" TYPE="char" LENGTH="16" NOTNULL="true" SEQUENCE="false" ENUM="true" ENUMVALUES="'qe', 'qb'" COMMENT="The type of this server - question engine or question bank." PREVIOUS="engineid" NEXT="url"/>
This generated an error when installing on MySQL, error message - default not a valid value for field type.
Looking at the SQL xmldb lib had produced, it had added default='' to the SQL statement. I got around this problem by specifying a (non null and non-empty/blank) default value in the install.xml - but there could be cases when you don't want a default specified in the xmldb definition - in which case xmldb lib shouldn't automatically add default=''