EPSG:6917

COMPD_CS["SVY21 + SHD height",
    GEOGCS["SVY21",
        DATUM["SVY21",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6757"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4757"]],
    VERT_CS["SHD height",
        VERT_DATUM["Singapore Height Datum",2005,
            AUTHORITY["EPSG","1140"]],
        UNIT["metre",1,
            AUTHORITY["EPSG","9001"]],
        AXIS["Gravity-related height",UP],
        AUTHORITY["EPSG","6916"]],
    AUTHORITY["EPSG","6917"]]

COMPOUNDCRS["SVY21 + SHD height",
    GEOGCRS["SVY21",
        DATUM["SVY21",
            ELLIPSOID["WGS 84",6378137,298.257223563,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        CS[ellipsoidal,2],
            AXIS["geodetic latitude (Lat)",north,
                ORDER[1],
                ANGLEUNIT["degree",0.0174532925199433]],
            AXIS["geodetic longitude (Lon)",east,
                ORDER[2],
                ANGLEUNIT["degree",0.0174532925199433]]],
    VERTCRS["SHD height",
        VDATUM["Singapore Height Datum"],
        CS[vertical,1],
            AXIS["gravity-related height (H)",up,
                LENGTHUNIT["metre",1]]],
    USAGE[
        SCOPE["Engineering survey, GIS, topographic mapping."],
        AREA["Singapore - onshore and offshore."],
        BBOX[1.13,103.59,1.47,104.07]],
    ID["EPSG",6917]]

GEOGCS["GCS_SVY21",
    DATUM["D_SVY21",
        SPHEROID["WGS_1984",6378137.0,298.257223563]],
    PRIMEM["Greenwich",0.0],
    UNIT["Degree",0.0174532925199433]],
VERTCS["SHD_height",
    VDATUM["Singapore_Height_Datum"],
    PARAMETER["Vertical_Shift",0.0],
    PARAMETER["Direction",1.0],
    UNIT["Meter",1.0]]

proj4.defs("EPSG:6917","+proj=longlat +ellps=WGS84 +vunits=m +no_defs +type=crs");

{
    "$schema": "https://proj.org/schemas/v0.7/projjson.schema.json",
    "type": "CompoundCRS",
    "name": "SVY21 + SHD height",
    "components": [
        {
            "type": "GeographicCRS",
            "name": "SVY21",
            "datum": {
                "type": "GeodeticReferenceFrame",
                "name": "SVY21",
                "ellipsoid": {
                    "name": "WGS 84",
                    "semi_major_axis": 6378137,
                    "inverse_flattening": 298.257223563
                }
            },
            "coordinate_system": {
                "subtype": "ellipsoidal",
                "axis": [
                    {
                        "name": "Geodetic latitude",
                        "abbreviation": "Lat",
                        "direction": "north",
                        "unit": "degree"
                    },
                    {
                        "name": "Geodetic longitude",
                        "abbreviation": "Lon",
                        "direction": "east",
                        "unit": "degree"
                    }
                ]
            }
        },
        {
            "type": "VerticalCRS",
            "name": "SHD height",
            "datum": {
                "type": "VerticalReferenceFrame",
                "name": "Singapore Height Datum"
            },
            "coordinate_system": {
                "subtype": "vertical",
                "axis": [
                    {
                        "name": "Gravity-related height",
                        "abbreviation": "H",
                        "direction": "up",
                        "unit": "metre"
                    }
                ]
            }
        }
    ],
    "scope": "Engineering survey, GIS, topographic mapping.",
    "area": "Singapore - onshore and offshore.",
    "bbox": {
        "south_latitude": 1.13,
        "west_longitude": 103.59,
        "north_latitude": 1.47,
        "east_longitude": 104.07
    },
    "id": {
        "authority": "EPSG",
        "code": 6917
    }
}

<?xml version="1.0" encoding="utf-8"?>
<Map srs="+proj=longlat +ellps=WGS84 +vunits=m +no_defs +type=crs">
	<Layer srs="+proj=longlat +ellps=WGS84 +vunits=m +no_defs +type=crs">
	</Layer>
</Map>

INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text, srtext) values ( 6917, 'EPSG', 6917, '+proj=longlat +ellps=WGS84 +vunits=m +no_defs +type=crs', 'COMPD_CS["SVY21 + SHD height",GEOGCS["SVY21",DATUM["SVY21",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6757"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4757"]],VERT_CS["SHD height",VERT_DATUM["Singapore Height Datum",2005,AUTHORITY["EPSG","1140"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Gravity-related height",UP],AUTHORITY["EPSG","6916"]],AUTHORITY["EPSG","6917"]]');