DB to JSON by Go.

main.go

package main

import (
	"encoding/json"
	"log"
	"os"

	"github.com/brianvoe/gofakeit/v6"
)

type UserValue struct {
	ID      int
	Name    string
	Address string
}

type User struct {
	ID        int      `json:"id"`
	Name      string   `json:"name"`
	Addresses []string `json:"addresses"`
}

func main() {
	var uvs []*UserValue
	for i := 0; i < 1000; i++ {
		id := i + 1
		name := gofakeit.Name()
		for j := 0; j < 100; j++ {
			uvs = append(uvs, &UserValue{
				ID:      id,
				Name:    name,
				Address: gofakeit.Address().Address,
			})
		}
	}

	m := map[int]*User{}
	for _, uv := range uvs {
		if _, ok := m[uv.ID]; !ok {
			m[uv.ID] = &User{
				ID:   uv.ID,
				Name: uv.Name,
			}
		}
		m[uv.ID].Addresses = append(m[uv.ID].Addresses, uv.Address)
	}

	var us []*User
	for _, v := range m {
		us = append(us, v)
	}

	if err := json.NewEncoder(os.Stdout).Encode(us); err != nil {
		log.Fatal(err)
	}
}