postgres_sql_instance can't get referenced even after I put it into a struct

[yingshaoxo]

Here is the code 1:

package database

import (
	"database/sql"
	"fmt"
	"log"

	_ "github.com/lib/pq" // add this
)

type FuckTheDatabaseClass struct {
	Postgres_sql *sql.DB
}

type TestUser struct {
	Email      string
	Username   *string
	Head_image *string //base64 string of an image
	Sex        *int32  //AI detect. 0: female, 1: male
	Age        *int32  //AI detect.
}

func (self FuckTheDatabaseClass) Init() error {
	postgres_sql_information := fmt.Sprintf(
		"host=%s port=%d user=%s password=%s dbname=%s sslmode=disable",
		"127.0.0.1", 5432,
		"yingshaoxo", "yingshaoxo",
		"postgres")
	db, err := sql.Open("postgres", postgres_sql_information)
	if err != nil {
		log.Fatal(err)
	}

	self.Postgres_sql = db
	//*self.Postgres_sql = *db

	return err
}

func (self FuckTheDatabaseClass) Print_all_data_in_a_table(table_name string) error {
	user := TestUser{}

	err := self.Postgres_sql.QueryRow("SELECT 'yingshaoxo', NULL").Scan(&user.Username, &user.Age)
        // error in here

	log.Println(user.Username)
	log.Println(user.Age)

	return err
}

When I call Print_all_data_in_a_table("user"), an error happen.

Error message is:

Exception has occurred: panic
"runtime error: invalid memory address or nil pointer dereference"
Stack:
	 2  0x00000001004efe14 in testing.tRunner.func1.2
	     at /opt/homebrew/Cellar/go/1.19.3/libexec/src/testing/testing.go:1396
	 3  0x00000001004ef710 in testing.tRunner.func1
	     at /opt/homebrew/Cellar/go/1.19.3/libexec/src/testing/testing.go:1399
	 7  0x0000000100554b8c in database/sql.(*DB).conn
	     at /opt/homebrew/Cellar/go/1.19.3/libexec/src/database/sql/sql.go:1288
	 8  0x0000000100556174 in database/sql.(*DB).query
	     at /opt/homebrew/Cellar/go/1.19.3/libexec/src/database/sql/sql.go:1730
	(truncated)

---

My questions are:

1. why the db variable sql.Open() returns are not copyable? If it is copyable, I could simply use it anywhere.
2. If you guys already implemented the whole PostgreSQL driver logic, why don't you guys make a new package to simplify the SQL development process? I mean, like what I did, to make the sql as an inner variable inside of a struct(class).