多亏了@jorge campos非常有帮助的评论，我才想出了一个可行的问题：

SELECT 
name.`field_site_sitelong_value` AS name,
basetable.`uuid`,
basetable.`nid`,
msl.`field_elevation_average_value`,
POLYFROMTEXT(concat(
        'Polygon((', 
            coordinates.`field_geo_bounding_box_left`   , ' ', coordinates.`field_geo_bounding_box_bottom`  , ', ',     
            coordinates.`field_geo_bounding_box_right`  , ' ', coordinates.`field_geo_bounding_box_bottom`  , ', ', 
            coordinates.`field_geo_bounding_box_right`  , ' ', coordinates.`field_geo_bounding_box_top`     , ', ',
            coordinates.`field_geo_bounding_box_left`   , ' ', coordinates.`field_geo_bounding_box_top`     , ', ', 
            coordinates.`field_geo_bounding_box_left`   , ' ', coordinates.`field_geo_bounding_box_bottom`  , 
        '))'
    ))
 AS GEOM
FROM `node` basetable
INNER JOIN `field_data_field_geo_bounding_box` coordinates
ON coordinates.`entity_id` = basetable.`nid`
INNER JOIN `field_data_field_site_sitelong` name
ON name.`entity_id` = basetable.`nid`
LEFT JOIN `field_data_field_elevation_average` msl
ON msl.`entity_id` = basetable.`nid`
WHERE basetable.`status` = 1 
  AND `field_geo_bounding_box_geo_type` = 'polygon'

这个 POLYFROMTEXT 函数从 concat 函数转换为实际的多边形空间数据类型。如果你忽略了 POLYFROMTEXT 您将获得每个多边形的可读wkt。
记住，在为多边形构造wkt时，点的顺序很重要 (POLYGON((left bottom,right bottom,right top,left top,left bottom))) ，并且第一个点和最后一个点必须相同，以便多边形闭合。