ST_ToLineString method for type ST

ST_ToLineString method for type ST_Geometry

Converts the geometry to a linestring.

Syntax

geometry-expression.ST_ToLineString()

Returns

ST_LineString If the geometry-expression is of type ST_LineString, return the geometry-expression. If the geometry-expression is of type ST_CircularString or ST_CompoundCurve, return geometry-expression.ST_CurveToLine(). If the geometry-expression is a geometry collection with a single element of type ST_Curve, return that element cast as ST_LineString. If the geometry-expression is the empty set, return an empty set of type ST_LineString. Otherwise, raise an exception condition.
The spatial reference system identifier of the result is the same as the spatial reference system of the geometry-expression.

Converts the geometry to a linestring. The logic is equivalent to that used for CAST( geometry-expression AS ST_LineString ). If the geometry-expression is a circular string or compound curve, it is approximated using ST_CurveToLine().

If geometry-expression is already known to be an ST_LineString value, it is more efficient to use TREAT( geometry-expression AS ST_LineString ) than the ST_ToLineString method.

Note

By default, ST_ToLineString uses the original format for a geometry, if it is available. Otherwise, the internal format is used. For more information about internal and original formats, see STORAGE FORMAT clause, CREATE SPATIAL REFERENCE SYSTEM statement.

Standards and compatibility

Example

The following returns an error because the Shape column is of type ST_Geometry and ST_Geometry does not support the ST_Length method.

SELECT Shape.ST_Length() 
FROM SpatialShapes WHERE ShapeID = 5

The following uses ST_ToLineString to change the type of the Shape column expression to ST_LineString. ST_Length returns the result 7.

SELECT Shape.ST_ToLineString().ST_Length() 
FROM SpatialShapes WHERE ShapeID = 5

In this case, the value of the Shape column is known be of type ST_LineString, so TREAT can be used to efficiently change the type of the expression. ST_Length returns the result 7.

SELECT TREAT( Shape AS ST_LineString ).ST_Length() 
FROM SpatialShapes WHERE ShapeID = 5

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