SQL:
外连接和内连接:
左连接或左外连接:包含左边的表的所有行,如果右边表中的某行没有匹配,该行内容为空(NULL)
--outer jion:left join or left outer join
select * from dbo.Project left join dbo.Voice on (dbo.Project.voiceID=dbo.Voice.ID)
右连接或右外连接:包含右边表的所有行,如果左边表中的某行没有匹配,该行内容为空(NULL)
--outer jion:right join or right outer join
select * from dbo.Project right join dbo.Voice on(dbo.Project.voiceID=dbo.Voice.ID)
完全连接或完全外连接:涵盖左连接与右连接,包含两张表中的所有行,不匹配的内容为空(NULL)
--outer join:full join or full outer join
select * from dbo.Project full join dbo.Voice on (dbo.Project.voiceID=dbo.Voice.ID)
内连接:
--join or inner join
select * from dbo.Project inner join dbo.Voice on (dbo.Project.voiceID=dbo.Voice.ID)
select * from dbo.Project,dbo.Voice where (dbo.Project.voiceID=dbo.Voice.ID)
select p.ID,p.Name,p.bak,v.NAME from dbo.Project p join dbo.Voice v on(p.voiceID=v.ID)
交叉连接:(1)分为没有where子句的交叉连接将产生连接所涉及的表的笛卡尔积。第一个表的行数乘以第二个表的行数等于笛卡尔积结果集的大小。
--cross join
select * from dbo.Project cross join dbo.Voice
select * from dbo.Project cross join dbo.Voice where (dbo.Project.voiceID=dbo.Voice.ID)
--两表关系为一对多,多对一,多对多的连接语句
select p.Name,v.NAME from dbo.Project_Voice as pv left join dbo.Voice as v on v.ID=pv.voiceID left join dbo.Project as p on p.ID=pv.projectID
select p.Name,v.NAME from dbo.Project_Voice as pv left join dbo.Voice as v on v.ID=pv.voiceID left join dbo.Project as p on p.ID=pv.projectID
where p.voiceID=v.ID
LINQ:(概念和结果参考sql语句的)
左连接:
var userInfo = from s in db.Project
join c in db.Voice on s.voiceID equals c.ID into ProjectV
from pv in ProjectV.DefaultIfEmpty()
select new { s.Name,s.bak,pv.NAME,s.ID};
右连接:
var userInfo = from c in db.Voice
join s in db.Project on c.ID equals s.voiceID into PV
from pv in PV.DefaultIfEmpty()
select new { pv.Name,pv.bak,pv.ID,c.NAME};
内连接:
var userInfo = from s in db.Project
join c in db.Voice on s.voiceID equals c.ID
select new { s.Name, s.bak, c.NAME, s.ID };