If a field name for sortby is defined, then this is ignored.
Otherwise this is used as the 'ORDER BY' statement to sort the records in the table when listed in the TYPO3 backend. It is possible to have multiple field names in here, and each can have an ASC or DESC keyword. Note that the value should not be prefixed with 'ORDER BY' in itself.
Sort by title:
'ctrl' => [ 'default_sortby' => 'title', ... ],
Sort by title and then by creation date:
'ctrl' => [ 'default_sortby' => 'title ASC, crdate DESC', ... ],
Do not confuse this property with sortby: default_sortby should be set only if there
is no sortby. The sortby field (typically set to
sorting) contains an integer
for explicit sorting , the backend then shows "up" and "down" buttons to manage sorting of records relative
to each other. The default_sortby should only be set if that explicit sorting is not wanted or useful. For
instance, the list of frontend users is sorted by username and has no other explicit sorting field in the database.