首先我们看下官方Go语言操作DB的示例：

// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sql_test

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

var db *sql.DB

func ExampleDB_Query() {
	age := 27
	rows, err := db.Query("SELECT name FROM users WHERE age=?", age)
	if err != nil {
		log.Fatal(err)
	}
	defer rows.Close()
	for rows.Next() {
		var name string
		if err := rows.Scan(&name); err != nil {
			log.Fatal(err)
		}
		fmt.Printf("%s is %d\n", name, age)
	}
	if err := rows.Err(); err != nil {
		log.Fatal(err)
	}
}

func ExampleDB_QueryRow() {
	id := 123
	var username string
	err := db.QueryRow("SELECT username FROM users WHERE id=?", id).Scan(&username)
	switch {
	case err == sql.ErrNoRows:
		log.Printf("No user with that ID.")
	case err != nil:
		log.Fatal(err)
	default:
		fmt.Printf("Username is %s\n", username)
	}
}

func ExampleDB_Query_multipleResultSets() {
	age := 27
	q := `
create temp table uid (id bigint); -- Create temp table for queries.
insert into uid
select id from users where age < ?; -- Populate temp table.

-- First result set.
select
	users.id, name
from
	users
	join uid on users.id = uid.id
;

-- Second result set.
select 
	ur.user, ur.role
from
	user_roles as ur
	join uid on uid.id = ur.user
;
	`
	rows, err := db.Query(q, age)
	if err != nil {
		log.Fatal(err)
	}
	defer rows.Close()

	for rows.Next() {
		var (
			id   int64
			name string
		)
		if err := rows.Scan(&id, &name); err != nil {
			log.Fatal(err)
		}
		fmt.Printf("id %d name is %s\n", id, name)
	}
	if !rows.NextResultSet() {
		log.Fatal("expected more result sets", rows.Err())
	}
	var roleMap = map[int64]string{
		1: "user",
		2: "admin",
		3: "gopher",
	}
	for rows.Next() {
		var (
			id   int64
			role int64
		)
		if err := rows.Scan(&id, &role); err != nil {
			log.Fatal(err)
		}
		fmt.Printf("id %d has role %s\n", id, roleMap[role])
	}
	if err := rows.Err(); err != nil {
		log.Fatal(err)
	}
}

我们需要了解的几种错误处理的场景：

Errors From Iterating Resultsets:

for rows.Next() {
	// ...
}
if err = rows.Err(); err != nil {
	// handle the error here
}

Errors From Closing Resultsets：

for rows.Next() {
	// ...
	break; // whoops, rows is not closed! memory leak...
}
// do the usual "if err = rows.Err()" [omitted here]...
// it's always safe to [re?]close here:
if err = rows.Close(); err != nil {
	// but what should we do if there's an error?
	log.Println(err)
}

Errors From QueryRow():

var name string
err = db.QueryRow("select name from users where id = ?", 1).Scan(&name)
if err != nil {
	log.Fatal(err)
}
fmt.Println(name)

上面的查询name很有可能没有值所以，最好使用下面的方式进行错误处理：

var name string
err = db.QueryRow("select name from users where id = ?", 1).Scan(&name)
if err != nil {
	if err == sql.ErrNoRows {
		// there were no rows, but otherwise no error occurred
	} else {
		log.Fatal(err)
	}
}
fmt.Println(name)

Identifying Specific Database Errors：

rows, err := db.Query("SELECT someval FROM sometable")
// err contains:
// ERROR 1045 (28000): Access denied for user 'foo'@'::1' (using password: NO)
if strings.Contains(err.Error(), "Access denied") {
	// Handle the permission-denied error
}

然而上面的处理并不是database/sql方式，我们应该用下面的方式处理：

if driverErr, ok := err.(*mysql.MySQLError); ok { // Now the error number is accessible directly
	if driverErr.Number == 1045 {
		// Handle the permission-denied error
	}
}

然而上面方式看起来依然很不清真，我们可以

if driverErr, ok := err.(*mysql.MySQLError); ok {
	if driverErr.Number == mysqlerr.ER_ACCESS_DENIED_ERROR {
		// Handle the permission-denied error
	}
}