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Poly1 是一个边界框，用于计算 f1 和 f2 在其边界内的面积。如果 f1 存在，但 f2 不存在，如何使此查询返回结果？现在，如果边界框内只有 f1，则查询不会返回任何结果。

SELECT ST_Transform(poly1.the_geom,3857) AS the_geom_webmercator, 
       ST_AREA(ST_Union(ST_Transform(ST_Intersection(f1.the_geom,poly1.the_geom),3857)))*.000247105381 AS acreage_of_1,
       ST_AREA(ST_Union(ST_Transform(ST_Intersection(f2.the_geom,poly1.the_geom),3857)))*.000247105381 AS acreage_of_2
            FROM poly1 JOIN farmland f1 ON f1.polygon_type = 'A' AND
                                           st_intersects(f1.the_geom,poly1.the_geom)
                       JOIN farmland f2 ON f2.polygon_type = 'B'
            GROUP BY poly1.the_geom
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1 回答 1

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您希望在“farmland f2”语句之前有一个左外连接。左外连接将保留所有行，即使 f2 中没有匹配项：

Select ST_Transform(poly1.the_geom,3857) as the_geom_webmercator, 
       ST_AREA(ST_Union(ST_Transform(ST_Intersection(f1.the_geom,poly1.the_geom),3857)))*.000247105381 AS acreage_of_1,
       ST_AREA(ST_Union(ST_Transform(ST_Intersection(f2.the_geom,poly1.the_geom),3857)))*.000247105381 AS acreage_of_2
FROM poly1 JOIN
     farmland f1
     ON f1.polygon_type = 'A' AND st_intersects(fp.the_geom,poly1.the_geom) **LEFT OUTER** JOIN
     farmland f2
     ON f2.polygon_type = 'B'
 group by poly1.the_geom
于 2012-07-16T19:40:52.973 回答