LINQ的左连接右连接内连接

Posted 2023-02-03 Nemo_XP

1、左连接：

var LeftJoin = from t1 in l1
join t2 in l2 on t1.ID equals t2.ID 
into Joinedt12
from t3 in Joinedt12.DefaultIfEmpty()
select new                        

Name = t1.Name,
Age=t2.Age                      
;

var list = from a in db.tableInfos1
           join b in db.tableInfos2 on a.ID equals b.Id into bb
           from bbdata in bb.DefaultIfEmpty()
           select new
           
               info1=a,
               info2 = bbdata ,
           ;

说明：新生成的列表list有两个属性info1，info2。其中info1对应的 tableInfos1 表，info2对应的 tableInfos2 表。

若要实现三个表左连接查询：

var list = from a in db.tableInfos1
           join b in db.tableInfo2 on a.ID equals b.Id into bb
           from bbdata in bb.DefaultIfEmpty()
           join c in db.tableInfos3 on a.EID equals c.Id into cc
           from ccdata in cc.DefaultIfEmpty()
           select new
           
                OccurrenceDate = bbdata.OccurrenceDate?.Replace("00:00", ""),//可对属性进行处理再赋值属性即可
                info1=a,
                info2 = bbdata ,
                info3 = ccdata
           ;

2、右连接：

var RightJoin = from t2 in l2
join t1in l1 on t2.ID equals t1.ID 
into joint12
from t3 in joint12.DefaultIfEmpty()
select new                          

Name = t1.Name,
Age=t2.Age  
;

3、内连接：

 var query = from t1 in l1
       join t2 in l2
       on t1.ID equals t2.ID
　　　　select new t3
      
        USER_ID = t1.USER_ID,
        USER_NAME = t1.USER_NAME,
        USER_PASSWORD = t1.USER_PASSWORD,
      ;

注：上例中使用了DefaultIfEmpty操作符，它能够为实序列提供一个默认的元素。DefaultIfEmpty使用了泛型中的default关键字。default关键字对于引用类型将返回null,而对于值类型则返回0。对于结构体类型，则会根据其成员类型将它们相应地初始化为null(引用类型)或0(值类型)

我们可以不使用default关键字，但在要DefaultIfEmpty中给定当空时的默认对象值。

多字段关联查询

var a = from m in cx.Set<A>()
join q in cx.Set<B>()
on new  Id = m.Id, EId = m.Ext  equals new  Id = q.BId, EId = q.BExt 
select new  m ;

cx.Set<A>().Join(cx.Set<B>(), outer => new Id = outer.Id, EId = outer.Ext,
inner => new Id = inner.BId, EId = inner.BExt,
(outer, inner) => new  );