ST_Area({poly|mpoly})

Returns a double-precision number indicating the area of the Polygon or
MultiPolygon argument, as measured in its spatial reference system.

As of MySQL 8.0.13, ST_Area() handles its arguments as described in the
introduction to this section, with these exceptions:

o If the geometry is geometrically invalid, either the result is an
  undefined area (that is, it can be any number), or an error occurs.

o If the geometry is valid but is not a Polygon or MultiPolygon object,
  an ER_UNEXPECTED_GEOMETRY_TYPE error occurs.

o If the geometry is a valid Polygon in a Cartesian SRS, the result is
  the Cartesian area of the polygon.

o If the geometry is a valid MultiPolygon in a Cartesian SRS, the
  result is the sum of the Cartesian area of the polygons.

o If the geometry is a valid Polygon in a geographic SRS, the result is
  the geodetic area of the polygon in that SRS, in square meters.

o If the geometry is a valid MultiPolygon in a geographic SRS, the
  result is the sum of geodetic area of the polygons in that SRS, in
  square meters.

o If an area computation results in +inf, an ER_DATA_OUT_OF_RANGE error
  occurs.

o If the geometry has a geographic SRS with a longitude or latitude
  that is out of range, an error occurs:

  o If any longitude argument is not in the range (−180, 180], an
    ER_LONGITUDE_OUT_OF_RANGE error occurs.

  o If any latitude argument is not in the range [−90, 90], an
    ER_LATITUDE_OUT_OF_RANGE error occurs.

  Ranges shown are in degrees. The exact range limits deviate slightly
  due to floating-point arithmetic.

Prior to MySQL 8.0.13, ST_Area() handles its arguments as described in
the introduction to this section, with these exceptions:

o For arguments of dimension 0 or 1, the result is 0.

o If a geometry is empty, the return value is 0 rather than NULL.

o For a geometry collection, the result is the sum of the area values
  of all components. If the geometry collection is empty, its area is
  returned as 0.

o If the geometry has an SRID value for a geographic spatial reference
  system (SRS), an ER_NOT_IMPLEMENTED_FOR_GEOGRAPHIC_SRS error occurs.

URL: https://dev.mysql.com/doc/refman/8.0/en/gis-polygon-property-functions.html

