猜想：以下两条SQL等价
select * from A left join  B on A.ID=B.BID and B.BName=N'小明'
select * from A left join (select * from B where B.BName=N'小明') B on A.ID=B.BID

为了方便验证，新建两张表A和Ｂ，然后插入６条数据到Ａ表,3条数据到B表。语句如下：
create table A(
    ID int identity(1,1) not null,
    AName nvarchar(300) null
)
create table B(
    ID int identity(1,1) not null,
    BID int  not null,
    BName nvarchar(300) null
)
insert into B(BName,BID) values(N'小明',2)
insert into B(BName,BID) values(N'小网',1)
insert into B(BName,BID) values(N'小刚',1)
insert into B(BName,BID) values(N'小蒋',1)
insert into B(BName,BID) values(N'小赵',1)
insert into B(BName,BID) values(N'小流',1)
insert into A(AName) values(N'一班')
insert into A(AName) values(N'二班')
insert into A(AName) values(N'三班')