What is the PostGIS / PostgreSQL equivalent of the [GeoPandas Overlay Union operation][1]?

    res_union = geopandas.overlay(df1, df2, how='union')

Say I have two tables, 1 green, 1 red with each with two rows with geometry (polygons):

[![enter image description here][2]][2]

Then, the objective is to get:

[![enter image description here][3]][3]

which is a table with 7 rows/ features. See the GeoPandas [docs][1] for more info. 

I've tried spatial [joins][4] and using the [ST_Union][5] and ST_Intersection to no avail. I managed to reproduce the equivalent of

    res_intersection = geopandas.overlay(df1, df2, how='intersection')

Which is the equivalent of:

[![enter image description here][6]][6]

using the following SQL command:

    SELECT
        table1.letter,
        table2.number,
        ST_Intersection(table1.geom,table2.geom)
    FROM 
        table1,
        table2
    WHERE
        ST_Intersects(table1.geom,table2.geom)





One solution is to create a table for the intersections and one for the symmetrical differences and than outer join. However I don't know how to create the Symmetrical Difference:

    SELECT
        test.hybas_nld.pfaf_id,
        test.gadm_nld.gid_1,
        ST_SymDifference(test.hybas_nld.geom,test.gadm_nld.geom)
    FROM 
        test.hybas_nld,
        test.gadm_nld
    WHERE
    	ST_????(test.hybas_nld.geom,test.gadm_nld.geom)

with the result of:
[![enter image description here][7]][7]

Possible [duplicate][8] with the exception that I would like to keep the polygons where only geometry of table 1 or 2 exists (symmetrical difference). 


What would be the simplest and fastest way to get to the final result (Union)?

  [1]: http://geopandas.org/set_operations.html
  [2]: https://i.sstatic.net/NKHRF.png
  [3]: https://i.sstatic.net/O53mR.png
  [4]: http://revenant.ca/www/postgis/workshop/joins.html
  [5]: https://postgis.net/docs/ST_Union.html
  [6]: https://i.sstatic.net/Hp5cv.png
  [7]: https://i.sstatic.net/VgF7s.png
  [8]: https://gis.stackexchange.com/questions/112498/postgis-overlay-style-union-not-dissolve-style