PostGIS query to build a GeoJSON FeatureCollection

postgis-to-geojson-feature-collection.sql

SELECT json_build_object(
    'type', 'FeatureCollection',
    'crs',  json_build_object(
        'type',      'name', 
        'properties', json_build_object(
            'name', 'EPSG:4326'  
        )
    ), 
    'features', json_agg(
        json_build_object(
            'type',       'Feature',
            'id',         {id}, -- the GeoJson spec includes an 'id' field, but it is optional, replace {id} with your id field
            'geometry',   ST_AsGeoJSON(geom)::json,
            'properties', json_build_object(
                -- list of fields
                'field1', field1,
                'field2', field2
            )
        )
    )
)
FROM yourtable;  --replace with your table name

How would it look like?
Merging the polygons?