linq查询

Linq给我们带来了很多的便利，不必时时刻刻写sql，可以让一个不懂T-SQL的程序猿也能实现简单编程，但是linq在效率上确实比不上SQL。数据量越大，效率问题就明显。

一般查询可以这样写

var v = (from c in en.T_AQI
                         where c.DatState.Equals("2") && c.SiteID == siteID
                         orderby c.DataTime descending  //排序
                         select c).Take(30).ToList();//Take和top的作用一样。ToList()就不用说了

也可以指定列查询

var v = (from c in en.T_AQI
                         where c.DatState.Equals("2") && c.SiteID == siteID
                         orderby c.DataTime descending  
                         select new {name = st.Name, age= st.Age}).Take(30).ToList();//列名是自己随意写的

也可以链接查询

var v = (from c in en.T_AQI
         from u in r.TabUser
                         where c.DatState.Equals("2") && c.SiteID == siteID
                         orderby c.DataTime descending  
                         select new{u.Name,c.Age,...}).Take(30).ToList();  //这个是多对多查询

var v = (from c in en.T_AQI
         from u in en.T_AQI
                         where c.DatState.Equals("2") && c.SiteID == c.SiteID
                         orderby c.DataTime descending  
                         select new{name=c.Name,age=c.Age}).Take(30).ToList();  //这个是自连接查询

var v = (from c in en.User
            join u in en.Role on c .RoleId equals u.ID into userlist
                         where c.DatState.Equals("2") && c.SiteID == c.SiteID
                         orderby c.DataTime descending  
                         select new{c.Name,c.Age,u.RoleName}).Take(30).ToList();  //这个是双向接查询

var v = (from c in en.T_AQI
         from u in r.TabUser
                         where c.DatState.Equals("2") && c.SiteID == siteID
                         orderby c.DataTime descending  
                         select u).Take(30).ToList();  //这个是一对多查询