Could't convert <nil> into type ...?

I tried to using database/sql for query database row into a Go type, my codes snippet following:

type User struct {
	user_id     int64
	user_name   string
	user_mobile string
	password    string
	email       interface{}
	nickname    string
	level       byte
	locked      bool
	create_time string
	comment     string  // convert <nil> to *string error 
}

func TestQueryUser(t *testing.T) {
	db := QueryUser(driverName, dataSourceName)

	stmtResults, err := db.Prepare(queryAll)
	defer stmtResults.Close()

	var r *User = new(User)

	arr := []interface{}{
		&r.user_id, &r.user_name, &r.user_mobile, &r.password, &r.email,
		&r.nickname, &r.level, &r.locked, &r.create_time, &r.comment,
	}

	err = stmtResults.QueryRow(username).Scan(arr...)

	if err != nil {
		t.Error(err.Error())
	}

	fmt.Println(r.email)

}

MySQL:

As your see, some fields that has NULL value, so I have to set interface{} type into User struct of Go, which convert NULL to nil.

--- FAIL: TestQueryUser (0.00s)
    user_test.go:48: sql: Scan error on column index 9: unsupported Scan, storing driver.Value type <nil> into type *string

Somebody has a better way？ or I must change the MySQL field and set its DEFAULT ' '

First the short answer : There is some types in sql package for example sql.NullString (for nullable string in your table and guess Nullint64 and NullBool and ... usage ) and you should use them in your struct.

The long one : There is two interface for this available in go , first is Scanner and the other is Valuer for any special type in database, (for example,I use this mostly with JSONB in postgres) you need to create a type, and implement this two(or one of them) interface on that type.

the scanner is used when you call Scan function. the data from the database driver, normally in []byte is the input and you are responsible for handling it. the other one, is used when the value is used as input in query. the result "normally" is a slice of byte (and an error) if you need to only read data, Scanner is enough, and vice-versa, if you need to write parameter in query the Valuer is enough

for an example of implementation, I recommend to see the types in sql package.

Also there is an example of a type to use with JSONB/JSON type in postgresql

// GenericJSONField is used to handle generic json data in postgres
type GenericJSONField map[string]interface{}


// Scan convert the json field into our type
func (v *GenericJSONField) Scan(src interface{}) error {
	var b []byte
	switch src.(type) {
	case []byte:
		b = src.([]byte)
	case string:
		b = []byte(src.(string))
	case nil:
		b = make([]byte, 0)
	default:
		return errors.New("unsupported type")
	}

	return json.Unmarshal(b, v)
}

// Value try to get the string slice representation in database
func (v GenericJSONField) Value() (driver.Value, error) {
	return json.Marshal(v)
}

driver value is often []byte but string and nil is acceptable. so this could handle null-able fields too.