ST_CompoundCurve
If the geometry-expression is of type ST_CompoundCurve, return the geometry-expression. If the geometry-expression is of type ST_LineString or ST_CircularString, return a compound curve containing one element, the geometry-expression. If the geometry-expression is a geometry collection with a single element of type ST_Curve, return that element cast as ST_CompoundCurve. If the geometry-expression is the empty set, return an empty set of type ST_CompoundCurve. 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 circularstring. The logic is equivalent to that used for CAST( geometry-expression AS ST_CompoundCurve ).
If geometry-expression is already known to be an ST_CompoundCurve value, it is more efficient to use TREAT( geometry-expression AS ST_CompoundCurve ) than the ST_ToCompound method.