ST_PointN-ApsaraDB RDS(RDS)-阿里云帮助中心

Returns the Nth point of a LineString or circular string.

Syntax

geometry ST_PointN(geometry aLinestring, integer n);

Parameter	Type	Description
`aLinestring`	geometry	The LineString or circular string to query.
`n`	integer	The 1-based index of the point to return. Negative values count from the end: `-1` is the last point, `-2` is the second-to-last, and so on.

Non-LineString input: Returns NULL if aLinestring is not a LineString or circular string (for example, a Polygon or MultiLineString).
Negative indexing: Negative values count backward from the end of the LineString. -1 returns the last point.
MultiLineString: To get the Nth point of each component LineString in a MultiLineString, combine ST_PointN with ST_Dump.
Circular strings and curves: ST_PointN supports circular strings and other curve types.
3D objects: ST_PointN preserves Z coordinates and does not drop them.

SELECT ST_AsText(ST_PointN('LINESTRING(0 0,2 2)'::geometry, 1));

Output:

 st_astext
------------
 POINT(0 0)
(1 row)

Use -1 to get the last point without knowing the total number of points.

SELECT ST_AsText(ST_PointN('LINESTRING(0 0,2 2)'::geometry, -1));

Output:

 st_astext
------------
 POINT(2 2)
(1 row)