The ST_MultiCurveAggr aggregate function can be used to combine a group of curves into a single collection. All of the geometries
to be combined must have both the same SRID and the same coordinate dimension.
Rows where the argument is NULL are not included.
Returns NULL for an empty group or a group containing no non-NULL values.
The resulting ST_MultiCurve has the same coordinate dimension as each curves.
The optional ORDER BY clause can be used to arrange the elements in a particular order so that ST_GeometryN returns them
in the desired order. If this ordering is not relevant, it is more efficient to not specify an ordering. In that case, the
ordering of elements depends on the access plan selected by the query optimizer.
ST_MultiCurveAggr is more efficient than ST_UnionAggr, but ST_MultiCurveAggr can return a collection with duplicate or overlapping
curves if they exist in the group of curves. ST_UnionAggr handles duplicate and overlapping geometries.
The following example returns a single value which combines all geometries of type ST_Curve from the SpatialShapes table into
a single collection of type ST_MultiCurve. If the Shape column was of type ST_Curve then the TREAT function and WHERE clause
would not be necessary.
SELECT ST_MultiCurve::ST_MultiCurveAggr( TREAT( Shape AS ST_Curve ) )
FROM SpatialShapes WHERE Shape IS OF( ST_Curve )