# ST_SymmetricDiff

## 定义

ST_SymmetricDiff 以两个 ST_Geometry 对象作为输入参数，返回由源对象的非公共部分组成的几何对象。

## 语法

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

ST_Geometry

## 示例

hazardous_sites 表在 ID 列中存储场地标识，而各场地的实际地理位置存储在场地点列中。

``````CREATE TABLE sensitive_areas (id integer,
shape sde.st_geometry);

CREATE TABLE hazardous_sites (id integer,
site sde.st_geometry);
``````
``````INSERT INTO SENSITIVE_AREAS VALUES (
1,
sde.st_geometry ('polygon ((20 30, 30 30, 30 40, 20 40, 20 30))', 0)
);

INSERT INTO SENSITIVE_AREAS VALUES (
2,
sde.st_geometry ('polygon ((30 30, 30 50, 50 50, 50 30, 30 30))', 0)
);

INSERT INTO SENSITIVE_AREAS VALUES (
3,
sde.st_geometry ('polygon ((40 40, 40 60, 60 60, 60 40, 40 40))', 0)
);

INSERT INTO HAZARDOUS_SITES VALUES (
4,
sde.st_geometry ('point (60 60)', 0)
);

INSERT INTO HAZARDOUS_SITES VALUES (
5,
sde.st_geometry ('point (30 30)', 0)
);
``````

ST_Buffer 函数生成一个环绕危险废弃物场地点的缓冲区域。ST_SymmetricDiff 函数将返回不相交的缓冲危险废弃物场地和敏感区域的面。

Oracle

``````SELECT sa.id SA_ID,hs.id HS_ID,
sde.st_area (sde.st_symmetricdiff (sde.st_buffer (hs.site, .1), sa.shape)) AREA_NO_INT
FROM HAZARDOUS_SITES hs, SENSITIVE_AREAS sa
WHERE hs.id = 4;

SA_ID    HS_ID   AREA_NO_INT

1        4    100.031393
2        4    400.031393
3        4    400.015697
``````

PostgreSQL

``````SELECT sa.id AS SA_ID, hs.id AS HS_ID,
sde.st_area (sde.st_symmetricdiff (sde.st_buffer (hs.site, .1), sa.shape)) AS "no intersection"
FROM hazardous_sites hs, sensitive_areas sa
WHERE hs.id = 4;

sa_id     hs_id   no intersection

1        4    100.031393502001
2        4    400.031393502001
3        4    400.01569751
``````

9/15/2013