Get a default value for null columns by implementing databse/sql.Scanner

main.go

package main

import (
	"database/sql"
	"errors"
	_ "github.com/mattn/go-sqlite3"
	"log"
	"os"
)

type DefaultString string

func (d *DefaultString) Scan(value interface{}) error {
	if value == nil {
		return nil
	}
	if vString, ok := value.(string); ok {
		*d = DefaultString(vString)
		return nil
	}
	return errors.New("not clever enough to convert other types")
  //but if we wanted to be, pkg/database/sql/convert.go shows the way
}

func main() {
	tableDefinition := `
  CREATE TABLE example (
   eid    INTEGER PRIMARY KEY,
   etext  TEXT(32));`

	insertQ := `INSERT INTO example (eid, etext) VALUES (?, ?);`
	insertQBlank := `INSERT INTO example (eid) VALUES (?);`
	selectQ := `SELECT etext FROM example WHERE eid = ?`

	dbFileName := "a_very_stupid_test.db"

	_ = os.Remove(dbFileName) //don't care if we succeed at deleting this 
	db, err := sql.Open("sqlite3", dbFileName)

	_, err = db.Exec(tableDefinition)
	if err != nil {
		log.Print(err.Error()) //blithely continue on if the table already exists
	}

	//insert with a null column
	_, err = db.Exec(insertQBlank, 1)
	if err != nil {
		log.Panic(err.Error())
	}

	//insert with a non-null column
	_, err = db.Exec(insertQ, 2, "this ain't the default text")
	if err != nil {
		log.Panic(err.Error())
	}

	for _, v := range [2]int{1, 2} {
		fromDB := DefaultString("this is the default text")
		err := db.QueryRow(selectQ, v).Scan(&fromDB)
		if err != nil {
			log.Panic(err.Error())
		}
		log.Printf("%d: %s", v, fromDB)
	}
}