Linq 基础查询

//基本查询
var query = from c in WorkFlows
             select c;

//带条件查询
var query = from c in WorkFlows
             where c.Pid == 1
             select c;

//查询显示不同的列
from c in WorkFlows 
select new
{
   c.ID,
   c.WorkFlowName
}

//排序
from c in WorkFlows
orderby c.ID descending
select c

//去除某个字段的重复
from c in WorkFlows
where c.Pid == 1
select c.Step).Distinct()

//带条件的分组查询
from c in WorkFlows
where c.Pid == 1
group c by c.Step into g
select g

from c in Cities
where c.State == "北京市"
select c

//分组后查询最大的 Min或最小的
from c in Cities 
group c by c.State into g
select new
{
   g.Key,         
   MaxId = g.Max(c => c.Id),
}

//查询包含数组中的数据
from c in Cities 
where (new string[] { "河南省", "北京市" }).Contains(c.State)
select c


from c in Cities 
orderby c.Id descending,c.Sz_code ascending
select c

//查询Content字段包含“西”的和字段State以“河”开头的数据 并连接
(from c in Cities 
where c.Content.Contains("西")
select c).Union
(from c in Cities 
where c.State.StartsWith("河") 
select c)

//子查询
from p in PersonTables 
select new
{
   ID = p.ID,
   CityID = (from c in Cities where c.Id == p.CityID select c.Content),
   PersonName=p.PersonName,
   Sex = p.Sex
}

//左连接查询 
from p in PersonTables
join c in Cities  on p.CityID equals c.Id
    into pro
    from x in pro.DefaultIfEmpty() 

//显示左边没有关联的数据，如果不用DefaultIfEmpty() 则不会显示左边表的全部数据
from x in pro
select new
{
   ID = p.ID,
   PersonName = p.PersonName,
   Sex = p.Sex,
   Content = x.Content==null ? "不存在" :x.Content
}

//多表关联join查询
from c in Cities 
join p in PersonTables  on c.Id equals p.CityID
    into cro
    from x in cro.DefaultIfEmpty()
join w in WorkFlows  on x.ID equals w.ID 
    into xrw 
    from s in xrw.DefaultIfEmpty()
select new
{
   Id = c.Id,
   State = c.State,
   Content = c.Content,
   SzCode = c.Sz_code,
   Name = x.PersonName,
   Sex = x.Sex,
   WorkFlowName= s.WorkFlowName,
   Step = s.Step
}