如何从c＃linq查询表中选择最大日期

Question

我在这里堆栈我需要某人的帮助。我想从Refno相同Refno的不同表中获取最大日期。 我现在的代码给了我actionrecords表中存在的所有日期。

我当前的代码看起来像

public IQueryable<EmployerDetails> _employerProfileOverview(string refNo, string organisationname, string tradingname) 
{ 
    var query = (from _employerdetails in _entities.Organisations 
       join _lasttransaction in _entities.LevyTransactions on _employerdetails.Refno equals _lasttransaction.Refno 
       select new EmployerDetails 
       { 
        RefNo = _employerdetails.Refno, 
        Surname = _employerdetails.Surname, 
        TradingName = _employerdetails.TradingName, 
        Status = _employerdetails.Status, 
        TransactionDate = _lasttransaction.TransactionDate 
       }); 
    if (string.IsNullOrEmpty(refNo).Equals(false)) 
     query = query.Where(x => x.RefNo.Contains(refNo.Trim())); 

    if (string.IsNullOrEmpty(organisationname).Equals(false)) 
     query = query.Where(x => x.Surname.Contains(organisationname.Trim())); 

    if (string.IsNullOrEmpty(tradingname).Equals(false)) 
     query = query.Where(x => x.TradingName.Contains(tradingname.Trim())); 

    return query; 
} 

而我出来就把这个样子

Refno  TradingName     Status TransactionDate 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2011-01-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-03-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-04-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-05-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-06-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-07-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-08-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-09-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-10-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-11-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2012-12-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-01-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-02-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-03-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-04-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-05-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-05-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-06-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-11-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2013-12-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-01-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-02-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-03-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-04-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-05-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-06-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-07-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-08-01 00:00:00.00 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-09-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-10-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-11-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2014-12-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2015-01-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2015-02-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2015-03-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2015-04-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2015-05-01 00:00:00.000 
L940732953 PSG KONSULT BPK LICHTENBURG  A 2015-06-01 00:00:00.000 
L940733035 BROUGHTON AND JONCK    A 2011-05-01 00:00:00.000 
L940733035 BROUGHTON AND JONCK    A 2011-05-01 00:00:00.000 
L940733035 BROUGHTON AND JONCK    A 2011-05-01 00:00:00.000 
L940733035 BROUGHTON AND JONCK    A 2011-05-01 00:00:00.000 
L940733035 BROUGHTON AND JONCK    A 2011-05-01 00:00:00.000 
L940733035 BROUGHTON AND JONCK    A 2011-05-01 00:00:00.000 
L940736574 COVERS FOODS CC     Y 2011-03-01 00:00:00.000 
L940736574 COVERS FOODS CC     Y 2011-03-01 00:00:00.000 
L940736574 COVERS FOODS CC     Y 2011-05-01 00:00:00.000 
L940736574 COVERS FOODS CC     Y 2011-05-01 00:00:00.000 
L940738141 TEAMWORX SHOPFITTERS (PTY) LTD A 2011-01-01 00:00:00.000 
L940738141 TEAMWORX SHOPFITTERS (PTY) LTD A 2011-02-01 00:00:00.000 
L940738141 TEAMWORX SHOPFITTERS (PTY) LTD A 2011-02-01 00:00:00.000 
L940738141 TEAMWORX SHOPFITTERS (PTY) LTD A 2011-04-01 00:00:00.000 
L940738141 TEAMWORX SHOPFITTERS (PTY) LTD A 2011-05-01 00:00:00.000 

我只是想显示最大（交易）仅在每次每Personal Data个人纪录。

请帮帮我吧。

Answer 1

您可以直接return语句之前添加以下行：

query = query.OrderByDescending(q => q.TransactionDate).First(); 

另一种选择是由物业功能MoreLinq使用最大，这将使得代码更优雅。

此外，你应该写：

if (!string.IsNullOrEmpty(refNo)) 

Answer 2

这里需要使用group by和使用Max扩展方法找到最大的交易日： -

var query = (from _employerdetails in _entities.Organisations 
       join _lasttransaction in _entities.LevyTransactions 
       on _employerdetails.Refno equals _lasttransaction.Refno 
       group new { _employerdetails, _lasttransaction } 
       by new { _employerdetails.Refno, 
         _employerdetails.Surname_employerdetails.TradingName, 
         _employerdetails.Status } into g 
       select new EmployerDetails 
       { 
        RefNo = g.Key.RefNo , 
        Surname = g.Key.Surname, 
        TradingName = g.Key.TradingName, 
        Status = g.Key.Status, 
        TransactionDate = g.Max(x => x._lasttransaction .TransactionDate) 
       }); 

此外，作为一个侧面说明，因为String.IsNullOrEmpty返回布尔值，不需要将其与false或true进行比较。您可以直接执行此操作： -

if (!string.IsNullOrEmpty(refNo)) 
     query = query.Where(x => x.RefNo.Contains(refNo.Trim()));