postgresql - Postgresql中如何正确使用GROUP BY函数

Question

下面的代码运行良好，除了我确定 threshold AS 的步骤。它不是计算在 polygon_dump AS 步骤中创建的每个栅格的计数，而是将所有栅格一起计算。我一直在尝试使用 GROUP BY，但收效甚微。

WITH
--  Select Features
    feat AS 
        (SELECT toid AS building_id,
                wkb_geometry AS geom 
                FROM buildings
        ),
    polygon_dump AS
        (SELECT (ST_DumpAsPolygons(ST_Clip(a.st_roughness,1,b.geom,-9999,true))).val AS polygon_vals,building_id AS building_id2
            FROM grosvenor_raster_roughness a, feat b
        ),
    threshold AS
        (SELECT Count(*) AS thres_val
            FROM polygon_dump
                WHERE polygon_vals >= 0 AND polygon_vals < 0.5
        GROUP BY building_id2
        ),
    b_stats AS
        (SELECT  building_id, (stats).*
            FROM (SELECT building_id, ST_SummaryStats(ST_Clip(a.st_roughness,1,b.geom,-9999,true)) AS stats
                FROM grosvenor_raster_roughness a
                    INNER JOIN feat b
                ON ST_Intersects(b.geom,a.st_roughness)
            ) AS foo
        )
--  Summarise statistics
    SELECT count As pixel_count,
           thres_val AS threshold_val,
           cast(thres_val as real)/cast(count as real)*100 AS percent_value,
           min AS min_pixel_val,
           max AS max_pixel_val,
           mean AS avg_pixel_val,
           stddev AS pixel_stddev
        FROM b_stats, threshold
            WHERE count > 0;

我得到以下结果:

红色的两列是正确的结果，我需要做什么才能只得到这些结果？

Answer 1

您正在进行交叉连接。您需要将带有 building_id 的列添加到您的 threshold CTE 中，以便您可以使用 b_stats 加入它。我不确定它应该是 LEFT 还是 INNER JOIN，所以我要使用 INNER。

WITH
--  Select Features
    feat AS 
        (SELECT toid AS building_id,
                wkb_geometry AS geom 
                FROM buildings
        ),
    polygon_dump AS
        (SELECT (ST_DumpAsPolygons(ST_Clip(a.st_roughness,1,b.geom,-9999,true))).val AS polygon_vals,building_id AS building_id2
            FROM grosvenor_raster_roughness a, feat b
        ),
    threshold AS
        (SELECT building_id2 AS building_id, Count(*) AS thres_val
            FROM polygon_dump
                WHERE polygon_vals >= 0 AND polygon_vals < 0.5
        GROUP BY building_id2
        ),
    b_stats AS
        (SELECT  building_id, (stats).*
            FROM (SELECT building_id, ST_SummaryStats(ST_Clip(a.st_roughness,1,b.geom,-9999,true)) AS stats
                FROM grosvenor_raster_roughness a
                    INNER JOIN feat b
                ON ST_Intersects(b.geom,a.st_roughness)
            ) AS foo
        )
--  Summarise statistics
    SELECT count As pixel_count,
           thres_val AS threshold_val,
           cast(thres_val as real)/cast(count as real)*100 AS percent_value,
           min AS min_pixel_val,
           max AS max_pixel_val,
           mean AS avg_pixel_val,
           stddev AS pixel_stddev
        FROM b_stats
        JOIN threshold USING(building_id)
            WHERE count > 0;