Returns the maximum X coordinate value of the geometry-expression. This is computed by comparing the X attribute of all points in the geometry.
For geographic spatial reference systems, the returned value corresponds to the first coordinate in the axis order. If the
axis order is lat/lon/a/m, the minimum value corresponds to the western boundary of the geometry-expression as returned by ST_LongWest and the maximum value corresponds to the eastern boundary of geometry-expression as returned by ST_LongEast. In round-Earth model, if geometry-expression crosses the date line, the minimum value will be greater than the maximum value. If the axis order is lon/lat/z/m, the minimum
value corresponds to southernmost point of geometry-expression as returned by ST_LatSouth and the maximum value corresponds to the northernmost point of geometry-expression as returned by ST_LatNorth.
If the geometry-expression is an empty geometry (ST_IsEmpty()=1), then this method returns NULL.