# ST_Difference

## 定义

ST_Difference 获取两个 ST_Geometry 对象，然后返回表示两个源对象之差的 ST_Geometry 对象。

## 语法

`sde.st_difference (g1 sde.st_geometry, g2 sde.st_geometry)`

ST_Geometry

## 示例

``````CREATE TABLE footprints (building_id integer,
footprint sde.st_geometry);

CREATE TABLE lots (lot_id integer,
lot sde.st_geometry);

INSERT INTO footprints (building_id, footprint) VALUES (
1,
sde.st_polygon ('polygon ((0 0, 0 10, 10 10, 10 0, 0 0))', 0)
);

INSERT INTO footprints (building_id, footprint) VALUES (
2,
sde.st_polygon ('polygon ((20 0, 20 10, 30 10, 30 0, 20 0))', 0)
);

INSERT INTO footprints (building_id, footprint) VALUES (
3,
sde.st_polygon ('polygon ((40 0, 40 10, 50 10, 50 0, 40 0))', 0)
);

INSERT INTO lots (lot_id, lot) VALUES (
1,
sde.st_polygon ('polygon ((-1 -1, -1 11, 11 11, 11 -1, -1 -1))', 0)
);

INSERT INTO lots (lot_id, lot) VALUES (
2,
sde.st_polygon ('polygon ((19 -1, 19 11, 29 9, 31 -1, 19 -1))', 0)
);

INSERT INTO lots (lot_id, lot) VALUES (
3,
sde.st_polygon ('polygon ((39 -1, 39 11, 51 11, 51 -1, 39 -1))', 0)
);
``````

Oracle

``````SELECT SUM (sde.st_area (sde.st_difference (lot, footprint)))
FROM FOOTPRINTS bf, LOTS
WHERE bf.building_id = lots.lot_id;

SUM(ST_AREA(ST_DIFFERENCE(LOT,FOOTPRINT)))

114
``````

PostgreSQL

``````SELECT SUM (sde.st_area (sde.st_difference (lot, footprint)))
FROM footprints bf, lots
WHERE bf.building_id = lots.lot_id;

sum

114
``````
9/15/2013