Custom sql.NullInt64 that handles json marshalling nicely, http://play.golang.org/p/w8VzxLGd-f

null_int_64.go

package main

import (
	"database/sql"
	"encoding/json"
	"errors"
	"log"
	"strconv"
)

type Foo struct {
	Id     int       `json:"id"`
	WorkId NullInt64 `json:"work_id,omitempty"`
}

type NullInt64 sql.NullInt64

func (i *NullInt64) MarshalJSON() ([]byte, error) {
	if !i.Valid {
		return nil, nil
	}
	return []byte(strconv.Itoa(int(i.Int64))), nil
}

func (i *NullInt64) UnmarshalJSON(b []byte) error {
	s := string(b)
	if v, err := strconv.Atoi(s); err != nil {
		i.Int64 = int64(v)
		return nil
	}
	if s == "null" {
		return nil
	}
	return errors.New("Invalid NullINt64: " + s)
}

func main() {
	f := &Foo{100, NullInt64{100, true}}
	b, err := json.Marshal(f)
	if err != nil {
		log.Fatal(err)
	}
	log.Println(string(b))
}

According to this part of the spec https://golang.org/ref/spec#Type_declarations, your declared NullInt64 type alias does not inherit Scan() and Values() methods of sql.NullInt64 because sql.NullInt64 is not interface type.