c# - 将关联和连接 SQL 查询转换为 LINQ

Question

请考虑这个查询：

SELECT DISTINCT(fld_09 + fld_10), fld_Switch, fld_Area
FROM   Mytbl t
WHERE  fld_Year = 2000
   AND fld_Month = 1
   AND fld_02 != '04'
   AND fld_Switch = 2
   AND (
           (fld_09 + fld_10) NOT IN 
                (SELECT DISTINCT(fld_09 + fld_10)
                 FROM  Mytbl q
                 WHERE  fld_Year = 2000
                 AND fld_Month = 1
                 AND fld_02 != '04'
                 AND fld_Switch = 1
                 AND t.fld_Area = q.fld_Area)
       )
GROUP BY fld_Area, fld_09, fld_10, fld_Switch

此查询在 1 秒内执行。我想用 linq 编写此查询。我编写了以下代码：

var subQuery= (from r in ent.Mytbl 
               where r.fld_Year == Year &&
                     r.fld_Month == Month &&
                     r.fld_Switch == 1 &&
                     r.fld_02 != "04"
               select r.fld_09 + r.fld_10;

var query = (from o in ent.Mytbl 
             let code = o.fld_09 + o.fld_10
             where o.fld_Year == Year &&
                   o.fld_Month == Month &&
                   o.fld_Switch == 2 &&
                   o.fld_F02 != "04" &&
                   subQuery.Contains(code) == false
             group o by new { o.fld_Area, o.fld_09, o.fld_10, o.fld_Switch }
             into gr
             select new
             {
                 a = gr.Key.fld_09 + gr.Key.fld_10,
                 gr.Key.fld_Area,c=gr.Count()
             }).Distinct();

这 2 个查询是否相等？

第二个查询在执行时需要 2 分钟。我的错误在哪里？

Answer 1

如果您还没有，请确保您使用LinqPad来调整此查询。我相信该let子句是您的性能打击。