Create a geojson from mutiples tables in PostrgreSQL and PostGIS

PostGis_to_Geojson.sql

SELECT json_build_object(
    'type', 'FeatureCollection',
    'crs',  json_build_object(
        'type',      'name', 
        'properties', json_build_object(
            'name', 'EPSG:4326'  
        )
    ),
    'features', json_agg(ST_AsGeoJSON(t.*)::json)
    )
FROM ( 
    SELECT
        /* Accept multiples fields */
        ta1.name,
        ta1.geom
    FROM
        table1 ta1
    WHERE
        ta1.id = $1
    UNION ALL
    SELECT
        Null, /* Null value is accepted */
        ta2.geom
    FROM
        table2 ta2
    WHERE
        ta2.id = $1
    UNION ALL
    SELECT
        ta3.name_in_other_column, /* The columns names doesn't have to match */
        ta3.geom
    FROM
        table3 ta3
    WHERE
        ta3.id = $1
) as t(name, geom);