Go 之sql.NullString 转化为string

写一个统计程序的时候，从表里查询gateway字段因为存在空的情况，所以在定义结构体的时候，设置为sql.NullString类型。

在起一个协程去查询到数据，然后放入到channel中，channel之前定义的是string类型的，所以需要转换一下。

代码实例：

package main

import (
	"bufio"
	"context"
	"database/sql"
	_"github.com/go-sql-driver/mysql"
	"fmt"
	"io"
	"os"
	"strings"
	"sync"
	"time"
)


var (
	db *sql.DB
	wg sync.WaitGroup
)

type smclog struct {
	charge string
	gateway sql.NullString
}

func initDB()(err error){
	ctx,cancel := context.WithTimeout(context.Background(),time.Second*20)
	defer cancel()
	dns := "root:1234@tcp(localhost:3306)/test"
	db,err = sql.Open("mysql",dns)
	if err !=nil{
		return err
	}
	err =db.PingContext(ctx)
	if err !=nil{
		return err
	}
	return nil
}

func readFile(filename string,mchan chan string){
	defer wg.Done()
	defer close(mchan)
	file,err := os.Open(filename)
	if err !=nil{
		fmt.Println("open file failed err:",err)
		return
	}
	defer file.Close()
	reader := bufio.NewReader(file)
	for{
		line,err:= reader.ReadString('\n')
		if err == io.EOF{
//这儿文件末尾有一个空行，所以不用去处理最后一行
			break
		}
		if err !=nil{
			fmt.Println("read failed:err:",err)
			return
		}
		line =strings.TrimSuffix(line,"\n")
		line =strings.TrimSpace(line)
		mchan <- line
	}
}

func QueryData(datachan chan string)  {
	defer wg.Done()
	defer close(datachan)
	sqlStr := "select charge,gateway from LOG202101"
	stmt ,err := db.Prepare(sqlStr)
	if err !=nil{
		fmt.Println("prepare failed err :",err)
		return
	}
	defer stmt.Close()
	rows ,err := stmt.Query()
	if err !=nil{
		fmt.Println("query failed err :",err)
		return
	}
	defer rows.Close()
	for rows.Next(){
		var s smclog
		err := rows.Scan(&s.charge,&s.gateway)
		if err !=nil{
			fmt.Printf("scan failed err:%v\n",err)
			return
		}

		datachan <- s.charge+"-"+s.gateway.String
	}
}



func main() {

	err := initDB()
	if err !=nil{
		fmt.Println("init db failed err :",err)
		return
	}
	fmt.Println("init db success...")

	mobileWb := make(chan string)
	mobileFee := make(chan string)
	mobileData := make(chan string)

	wg.Add(3)
	go readFile("./mobile.txt",mobileWb)
	go readFile("./fee_mobile.txt",mobileFee)
	go QueryData(mobileData)


	mwb := make(map[string]int)
	for v :=range mobileWb{
		mwb[v] =1
	}

	mdb := make(map[string]string)
	for v:= range mobileData{
		sd := strings.Split(v,"-")
		mdb[sd[0]]= sd[1]
	}

	//mfee :=make(map[string]int)
	//for v1:=range mobileFee{
	//	mfee[v1]=1
	//}

	for k,_ := range mdb {
		_,ok := mwb[k]
		if ok{
			data := mdb[k]
			//返回网关和对应的手机号
			fmt.Println(data,k)
		}
	}

	wg.Wait()
}