There may be occasions where you need to search a specific field in Solr where the field has non-null values, equivalent to:

SELECT field_name FROM my_table WHERE field_name IS NOT NULL;

Solr equivalent:

FieldName:['' TO *]

You can also do the same for numeric ranges:

SELECT field_value FROM my_table WHERE field_value > 0;

Solr equivalent:

FieldValue:[1 TO *]

As always, it's best to try these types of queries via the admin interface before adding to code that actually constructs the Solr query.