{
    "name": "Id",
    "type": "INTEGER",
    "mode": "nullable"
  },
  {
    "name": "Address",
    "type": "RECORD",
    "mode": "repeated",
    "fields":[
      {
        "name": "Street",
        "type": "STRING",
        "mode": "nullable"
      },
      {
        "name": "City",
        "type": "STRING",
        "mode": "nullable"
      }
     ]
   }

table_schema = bigquery.TableSchema()

Id_schema = bigquery.TableFieldSchema()
Id_schema.name = 'Id'
Id_schema.type = 'INTEGER'
Id_schema.mode = 'nullable'
table_schema.fields.append(Id_schema)

Address_schema = bigquery.TableFieldSchema()
Address_schema.name = 'Address'
Address_schema.type = 'RECORD'
Address_schema.mode = 'repeated'

Street_schema = bigquery.TableFieldSchema()
Street_schema.name = 'Street'
Street_schema.type = 'STRING'
Street_schema.mode = 'nullable'
Address_schema.fields.append(Street_schema)
table_schema.fields.append(Address_schema)

City_schema = bigquery.TableFieldSchema()
City_schema.name = 'City'
City_schema.type = 'STRING'
City_schema.mode = 'nullable'
Address_schema.fields.append(City_schema)
table_schema.fields.append(Address_schema)

{"Id": 1, "Address": {"Street":"MG Road","City":"Pune"}}
{"Id": 2, "Address": {"City":"Mumbai"}}
{"Id": 3, "Address": {"Street":"XYZ Road"}}
{"Id": 4}
{"Id": 5, "PhoneNumber": 12345678, "Address": {"Street":"ABCD Road", "City":"Bangalore"}}

#standardSQL
WITH t AS (
  SELECT '{"Id": 1, "Address": {"Street":"MG Road","City":"Pune"}}' line UNION ALL
  SELECT '{"Id": 2, "Address": {"City":"Mumbai"}}' UNION ALL
  SELECT '{"Id": 3, "Address": {"Street":"XYZ Road"}}' UNION ALL
  SELECT '{"Id": 4}  ' UNION ALL
  SELECT '{"Id": 5, "PhoneNumber": 12345678, "Address": {"Street":"ABCD Road", "City":"Bangalore"}}'
)
SELECT
  JSON_EXTRACT_SCALAR(line, '$.Id') id,
  JSON_EXTRACT_SCALAR(line, '$.PhoneNumber') PhoneNumber,
  JSON_EXTRACT_SCALAR(line, '$[Address].Street') Street,
  JSON_EXTRACT_SCALAR(line, '$[Address].City') City
FROM t

Row id  PhoneNumber Street      City
1   1   null        MG Road     Pune
2   2   null        null        Mumbai
3   3   null        XYZ Road    null
4   4   null        null        null
5   5   12345678    ABCD Road   Bangalore