ST_AsGeoJSON(g [, max_dec_digits [, options]])

Generates a GeoJSON object from the geometry g. 从几何体g生成GeoJSON对象。The object string has the connection character set and collation.对象字符串具有连接字符集和排序规则。

If any argument is NULL, the return value is NULL. 如果任何参数为NULL，则返回值为NULL。If any non-NULL argument is invalid, an error occurs.如果任何非NULL参数无效，则会发生错误。

max_dec_digits, if specified, limits the number of decimal digits for coordinates and causes rounding of output. max_dec_digits（如果指定）限制坐标的小数位数，并导致输出舍入。If not specified, this argument defaults to its maximum value of 2³² − 1. 如果未指定，则此参数默认为其最大值2³²-1。The minimum is 0.最小值为0。

options, if specified, is a bitmask. options（如果指定）是位掩码。The following table shows the permitted flag values. 下表显示了允许的标志值。If the geometry argument has an SRID of 0, no CRS object is produced even for those flag values that request one.如果geometry参数的SRID为0，则不会生成CRS对象，即使对于那些请求CRS对象的标志值也是如此。

mysql> SELECT ST_AsGeoJSON(ST_GeomFromText('POINT(11.11111 12.22222)'),2);
+-------------------------------------------------------------+
| ST_AsGeoJSON(ST_GeomFromText('POINT(11.11111 12.22222)'),2) |
+-------------------------------------------------------------+
| {"type": "Point", "coordinates": [11.11, 12.22]}            |
+-------------------------------------------------------------+

ST_GeomFromGeoJSON(str [, options [, srid]])

Parses a string str representing a GeoJSON object and returns a geometry.解析表示GeoJSON对象的字符串str并返回几何体。

If any argument is NULL, the return value is NULL. 如果任何参数为NULL，则返回值为NULL。If any non-NULL argument is invalid, an error occurs.如果任何非空参数无效，则会发生错误。

options, if given, describes how to handle GeoJSON documents that contain geometries with coordinate dimensions higher than 2. options（如果给定）描述如何处理包含坐标维度大于2的几何体的GeoJSON文档。The following table shows the permitted options values.下表显示了允许的options值。

options values of 2, 3, and 4 currently produce the same effect. options值2、3和4当前会产生相同的效果。If geometries with coordinate dimensions higher than 2 are supported in the future, you can expect these values to produce different effects.如果将来支持坐标标注大于2的几何图形，则可以预期这些值会产生不同的效果。

The srid argument, if given, must be a 32-bit unsigned integer. srid参数（如果给定）必须是32位无符号整数。If not given, the geometry return value has an SRID of 4326.如果未给定，则几何体返回值的SRID为4326。

If srid refers to an undefined spatial reference system (SRS), an ER_SRS_NOT_FOUND error occurs.如果srid引用未定义的空间参考系（SRS），则会发生ER_SRS_NOT_FOUND错误。

For geographic SRS geometry arguments, if any argument has a longitude or latitude that is out of range, an error occurs:对于地理SRS几何参数，如果任何参数的经度或纬度超出范围，则会发生错误：

Ranges shown are in degrees. 显示的范围以度为单位。If an SRS uses another unit, the range uses the corresponding values in its unit. 如果SRS使用另一个单位，则范围使用其单位中的相应值。The exact range limits deviate slightly due to floating-point arithmetic.由于采用浮点运算，精确的范围限制略有偏差。

GeoJSON geometry, feature, and feature collection objects may have a crs property. GeoJSON几何体、要素和要素集合对象可能具有crs属性。The parsing function parses named CRS URNs in the urn:ogc:def:crs:EPSG::srid and EPSG:srid namespaces, but not CRSs given as link objects. 解析函数解析urn:ogc:def:crs:EPSG::srid和EPSG:srid命名空间中的命名CRS URNs，但不解析作为链接对象提供的CRS。Also, urn:ogc:def:crs:OGC:1.3:CRS84 is recognized as SRID 4326. 此外，urn:ogc:def:crs:OGC:1.3:CRS84被识别为SRID 4326。If an object has a CRS that is not understood, an error occurs, with the exception that if the optional srid argument is given, any CRS is ignored even if it is invalid.如果一个对象有一个不可理解的CRS，则会发生一个错误，除非给出了可选的srid参数，否则任何CRS都会被忽略，即使它是无效的。

If a crs member that specifies an SRID different from the top-level object SRID is found at a lower level of the GeoJSON document, an ER_INVALID_GEOJSON_CRS_NOT_TOP_LEVEL error occurs.如果在GeoJSON文档的较低级别找到指定与顶级对象SRID不同的SRID的crs成员，则会发生ER_INVALID_GEOJSON_CRS_NOT_TOP_LEVEL错误。

As specified in the GeoJSON specification, parsing is case-sensitive for the type member of the GeoJSON input (Point, LineString, and so forth). 正如GeoJSON规范中所指定的，解析对GeoJSON输入的type成员（Point、LineString等）是区分大小写的。The specification is silent regarding case sensitivity for other parsing, which in MySQL is not case-sensitive.该规范对其他解析的大小写敏感度保持沉默，而在MySQL中则不区分大小写。

This example shows the parsing result for a simple GeoJSON object. 此示例显示了一个简单GeoJSON对象的解析结果。Observe that the order of coordinates depends on the SRID used.请注意，坐标的顺序取决于所使用的SRID。

mysql> SET @json = '{ "type": "Point", "coordinates": [102.0, 0.0]}';
mysql> SELECT ST_AsText(ST_GeomFromGeoJSON(@json));
+--------------------------------------+
| ST_AsText(ST_GeomFromGeoJSON(@json)) |
+--------------------------------------+
| POINT(0 102)                         |
+--------------------------------------+
mysql> SELECT ST_SRID(ST_GeomFromGeoJSON(@json));
+------------------------------------+
| ST_SRID(ST_GeomFromGeoJSON(@json)) |
+------------------------------------+
|                               4326 |
+------------------------------------+
mysql> SELECT ST_AsText(ST_SRID(ST_GeomFromGeoJSON(@json),0));
+-------------------------------------------------+
| ST_AsText(ST_SRID(ST_GeomFromGeoJSON(@json),0)) |
+-------------------------------------------------+
| POINT(102 0)                                    |
+-------------------------------------------------+