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19.5.2.3. LineString函数

LineString由Point值组成。你可以提取LineString的特定点，计数它所包含的点数，或获取其长度。

• EndPoint(ls)

  返回LineString值1s的最后一个点的Point。

  mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
  mysql> SELECT AsText(EndPoint(GeomFromText(@ls)));
  +-------------------------------------+
  | AsText(EndPoint(GeomFromText(@ls))) |
  +-------------------------------------+
  | POINT(3 3)                          |
  +-------------------------------------+
  

• GLength(ls)

  以双精度数值返回LineString值1s在相关的空间参考系中的长度。

  mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
  mysql> SELECT GLength(GeomFromText(@ls));
  +----------------------------+
  | GLength(GeomFromText(@ls)) |
  +----------------------------+
  |            2.8284271247462 |
  +----------------------------+
  

• NumPoints(ls)

  返回LineString值1s中的点数。

  mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
  mysql> SELECT NumPoints(GeomFromText(@ls));
  +------------------------------+
  | NumPoints(GeomFromText(@ls)) |
  +------------------------------+
  |                            3 |
  +------------------------------+
  

• PointN(ls,n)

  返回LineString值1s中的第n个点。点编号从1开始。

  mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
  mysql> SELECT AsText(PointN(GeomFromText(@ls),2));
  +-------------------------------------+
  | AsText(PointN(GeomFromText(@ls),2)) |
  +-------------------------------------+
  | POINT(2 2)                          |
  +-------------------------------------+
  

• StartPoint(ls)

  返回LineString值1s的第一个点的Point。

  mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
  mysql> SELECT AsText(StartPoint(GeomFromText(@ls)));
  +---------------------------------------+
  | AsText(StartPoint(GeomFromText(@ls))) |
  +---------------------------------------+
  | POINT(1 1)                            |
  +---------------------------------------+
  

OpenGIS规范还定义了下述函数，MySQL尚未实施这些函数：

• IsRing(ls)

  如果LineString值ls是封闭的（即其StartPoint()和EndPoint()值相同）和简单的（未通过相同的点1次以上）返回1。如果ls不是环，返回0，如果它是NULL，返回-1。