.net - 将两个 LINQ 查询合并为一个

Question

我有一个表，其中大约有 15 个不同的字段，其中一些是 JobID(整数字段)、Cost(整数字段)和 LastUpdated(日期时间字段)

JobID  Cost      LastUpdated
 1      10        10-July-2011
 1      15        11-June-2011
 2      25        5-May-2011
 1      15        12-April-2011

Is it possible to write one LINQ query to get the sum of Cost for job id = 1 and also the last date such a cost incurred?

The example query output for the sample data above to look like this below:

40 , 10-july-2011

Currently I am doing it with two different linq queries like this, resulting in two hits to the database from the website in this particular case.

//for last updated
(from row in MyTable where row.JobID == 1
 orderby row.LastUpdated descending
 select row.LastUpdated).First()

//for sum of cost
(from row in MyTable  
 where row.JobID == 1
 select row.Cost).Sum()

在这种情况下，一个 linq 查询会更好吗？在整个页面加载过程中类似的情况将导致对数据库的多次命中，总共大约 18 次，针对 9 个不同的表。在将总和和 LastUpdated 日期合并到一个查询中时，我试图将命中计数减少到 9，每个表一个。

感谢您的宝贵时间...

Answer 1

是的，你可以像这样groupby

var query = from row in MyTable
            group row by row.JobID into rows
            where rows.Key == 1 
            select new
            {
              LastUpdated = rows.Select(x => x.LastUpdated).OrderByDescending(x => x).First(),
              Cost = rows.Sum(x => x.Cost),
            };