Question

我正在尝试从数据库中获取三列，DecalExpireDate，DecalExpireMonth和DecalExpireYear。基本上，我正在尝试检查它是否有一个以前的到期日期，如果有，我需要获得最新的到期日期，因为它可能有多个先前的到期日期。

var previousExpirationDate = (from d in db.CT_Decals
      where d.TankID == decal.TankID
      && d.DecalStatus == "Approved"
      && d.DecalExpireDate == ((from dn in db.CT_Decals
                               where dn.TankID == decal.TankID
                               && dn.DecalStatus == "Approved"
                               select dn.DecalExpireDate).Max())
      select new 
      {
        d.DecalExpireDate,
        d.DecalExpireMonth,
        d.DecalExpireYear

      });

此查询无效，有人可以看到问题吗？我正在通过TankID搜索，状态=已批准，然后我尝试使用max expirationDate。

Answer 1

这个怎么样

 var result = db.CT_Decals.Where(o => o.TankId ==  decal.TankId && o.DecalStatus == "Approved")
                  .OrderByDescending(o => o.DecalExpireDate)
                  .Select(o => new { o.DecalExpireDate,
                                     o.DecalExireMonth,
                                     o.DecalExireYear }).First()

或

var result = (from d in db.CT_Decals
                      where d.TankID == 1 && d.Status == "Approved"
                      orderby d.ExpireDate descending
                      select new { d.DecalExpireDate, 
                                   d.DecalExpireMonth, 
                                   d.ExpireDate }).First();

Answer 2

这个max函数的问题无法在你需要的范围内转换为Sql，所以你可以尝试这样做

 var previousExpirationDate = (from d in db.CT_Decals
  where d.TankID == decal.TankID
  && d.DecalStatus == "Approved"
  && d.DecalExpireDate == ((from dn in db.CT_Decals
                           where dn.TankID == decal.TankID
                           && dn.DecalStatus == "Approved"
                           order by dn.DecalExpireDate
                           select dn.DecalExpireDate).First())
  select new 
  {
    d.DecalExpireDate,
    d.DecalExpireMonth,
    d.DecalExpireYear

  });

Answer 3

虽然Max可能是您示例中的问题，但找到正确使用它的方法会很好，因为它比排序更快。怎么样：

编辑：此答案仅返回DecalExpireDate <击>

<击>

var previousExpirationDate = (from d in db.CT_Decals
      where d.TankID == decal.TankID
      && d.DecalStatus == "Approved"
      select new 
      {
        d.DecalExpireDate,
        d.DecalExpireMonth,
        d.DecalExpireYear
      }).Max(d => d.DecalExpireDate);

对于此版本，您必须进行测试，以确定是否值得对数据库进行额外调用以避免排序：

var latestDate = (from d in db.CT_Decals
      where d.TankID == decal.TankID
      && d.DecalStatus == "Approved"
      select d.DecalExpireDate).Max();

var previousExpirationDate = (from d in db.CT_Decals
      where d.TankID == decal.TankID
      && d.DecalStatus == "Approved"
      && d.DecalExpireDate == latestDate
      select new 
      {
        d.DecalExpireDate,
        d.DecalExpireMonth,
        d.DecalExpireYear
      }).First();

linq查询，如果存在上一个日期，则按最大日期搜索

3 个答案: