Example for joomla xml field type com_fileds aka jcfileds

jcfields.xml

                <field
                        name="field_title"
                        type="sql"
                        label="Select a field for title"
                        query="SELECT id, title FROM #__fields WHERE context LIKE '%com_content.article%' AND state=1"
                        key_field="id"
                        value_field="title"
                        required="true"
                >

query="SELECT id, title FROM #__fields WHERE context LIKE '%com_content.article%' AND state=1"
для показа только включенных полей