linq查詢

Linq給我們帶來了很多的便利，不必時時刻刻寫sql，可以讓一個不懂T-SQL的程序猿也能實現簡單編程，但是linq在效率上確實比不上SQL。數據量越大，效率問題就明顯。

一般查詢可以這樣寫

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的作用一樣。ToList()就不用說了

也可以指定列查詢

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();//列名是自己隨意寫的

也可以鏈接查詢

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();  //這個是多對多查詢

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();  //這個是自連接查詢

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();  //這個是雙向接查詢

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();  //這個是一對多查詢