sql：列索引 19 上的扫描错误，名称“L2Name”：不支持扫描，将驱动程序.Value 类型 <nil> 存储到 *string 类型中

Question

使用 Golang 和内置的 database/sql 库和 postgres lib/pq 库，我试图从某些记录中包含一些空值的数据库中读取。代码可以编译，但是当我尝试运行它时，出现以下错误： sql: Scan error on column index 19, name "L2Name": unsupported Scan, storing driver.Value type <nil> into type *string

我有这样的结构：

// Assets Info Dataset
type AssetInfo struct {
  Asset_id string
  Asset_name string
  Organisation_id string
  LastCheckIn string
  Asset_Status string
  Asset_latitude string
  Asset_longitude string
  Organisation_name string
  CurrentDevice_name string
  AssetActiveDeviceType string
  AssetSafetyTimer float32
  TemplateName string
  TemplateL2name string
  TemplateL2contact string
  TemplateL3name string
  TemplateL3contact string
  TemplateL4name string
  TemplateL4contact string
  TemplateEscalationNotes string
}

这是我的代码：

db, err := sql.Open("mysql", "stevejc:19939c@tcp(127.0.0.1:3306)/johntcw_loneworker?charset=utf8")
  checkErr(err)
  defer db.Close()

  for {
    sqlstatement := "SELECT" +
    " assets.ID, assets.Name, assets.LastCheckIn, assets.Status, assets.OffTimer," +
    " assets.SafetyTimer, assets.HazardTimer, assets.HazardTimerStartedTime, assets.LastSignedOn," +
    " assets.Latitude, assets.Longitude, assets.TemplateID, assets.ActiveDeviceType, assets.CurrentDeviceID," +
    " assets.OffTimerTemp, assets.OrganisationID," +
    " organisations.Name As OrganisationName," +
    " devices.Label As CurrentDeviceName," +
    " templates.Name As TemplateName, templates.L2Name, templates.L2Contact, templates.L3Name, templates.L3Contact," +
    " templates.L4Name, templates.L4Contact, templates.Note" +
    " FROM assets" +
    " LEFT JOIN organisations ON assets.OrganisationID = organisations.ID" +
    " LEFT JOIN devices ON assets.CurrentDeviceID = devices.ID" +
    " JOIN templates ON assets.TemplateID = templates.ID" +
    " WHERE assets.Status != 'Not monitoring' AND assets.AssetStatus = 'Active' AND assets.Display != '0'"
    // select monitoring assets
    rows, err := db.Query(sqlstatement)
    checkErr(err)

    for rows.Next() {
      var assetid string
      var name string
      var lastcheckin string
      var status string
      var offtimer float32
      var offtimertemp float32
      var safetytimer float32
      var hazardtimer float32
      var hazardstarttime string
      var lastsignedon string
      var lat string
      var lon string
      var templateid string
      var activedevicetype string
      var currentdeviceid string
      var organisationid string
      var organisationname string
      var currentdevicename string
      var templatename string
      var l2name string
      var l2contact string
      var l3name string
      var l3contact string
      var l4name string
      var l4contact string
      var escalationnotes string


      err = rows.Scan(&assetid,
        &name,
        &lastcheckin,
        &status,
        &offtimer,
        &safetytimer,
        &hazardtimer,
        &hazardstarttime,
        &lastsignedon,
        &lat,
        &lon,
        &templateid,
        &activedevicetype,
        &currentdeviceid,
        &offtimertemp,
        &organisationid,
        &organisationname,
        &currentdevicename,
        &templatename,
        &l2name,
        &l2contact,
        &l3name,
        &l3contact,
        &l4name,
        &l4contact,
        &escalationnotes)
      checkErr(err)

      assetinfo := new(AssetInfo)
      assetinfo.Asset_id = assetid
      assetinfo.Asset_name = name
      assetinfo.LastCheckIn = lastcheckin
      assetinfo.Asset_Status = status
      assetinfo.Organisation_id = organisationid
      assetinfo.Asset_longitude = lon
      assetinfo.Asset_latitude = lat
      assetinfo.Organisation_name = organisationname
      assetinfo.CurrentDevice_name = currentdevicename
      assetinfo.AssetActiveDeviceType = activedevicetype
      assetinfo.AssetSafetyTimer = safetytimer
      assetinfo.TemplateName = templatename
      assetinfo.TemplateL2name = l2name
      assetinfo.TemplateL2contact = l2contact
      assetinfo.TemplateL3name = l3name
      assetinfo.TemplateL3contact = l3contact
      assetinfo.TemplateL4name = l4name
      assetinfo.TemplateL4contact = l4contact
      assetinfo.TemplateEscalationNotes = escalationnotes

代码的输出是ok的，我可以从数据库中得到我想要的所有数据。除了 SQL 错误也打印在控制台上。并且这里的 L2Name 不是空值，我可以在控制台上打印该值。所以不知道为什么显示类型的错误？

Answer 1

最简单的解决方法是使用 COALESCE(templates.L2Name, '') 将可空列包装在 sql 语句中，感谢 @pmk 的帮助。

  sqlstatement := "SELECT" +
    " assets.ID, assets.Name, assets.LastCheckIn, assets.Status, assets.OffTimer," +
    " assets.SafetyTimer, assets.HazardTimer, assets.HazardTimerStartedTime, assets.LastSignedOn," +
    " assets.Latitude, assets.Longitude, assets.TemplateID, assets.ActiveDeviceType, assets.CurrentDeviceID," +
    " assets.OffTimerTemp, assets.OrganisationID," +
    " organisations.Name As OrganisationName," +
    " devices.Label As CurrentDeviceName," +
    " templates.Name As TemplateName, COALESCE(templates.L2Name, ''), COALESCE(templates.L2Contact, '')," +
    " COALESCE(templates.L3Name, ''), COALESCE(templates.L3Contact, '')," +
    " COALESCE(templates.L4Name, ''), COALESCE(templates.L4Contact, ''), templates.Note" +
    " FROM assets" +
    " LEFT JOIN organisations ON assets.OrganisationID = organisations.ID" +
    " LEFT JOIN devices ON assets.CurrentDeviceID = devices.ID" +
    " JOIN templates ON assets.TemplateID = templates.ID" +
    " WHERE assets.Status != 'Not monitoring' AND assets.AssetStatus = 'Active' AND assets.Display != '0'"

Answer 2

对于可以为 NULL 的字段，您应该使用 sql.NullString 数据结构。 这里有更多细节：

https://golang.org/pkg/database/sql/#NullString