Returns the maximum X coordinate value of the geometry-expression. This is computed by comparing the X attribute of all points in the geometry.
Note that 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, this means that 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.