--> 测试数据：@User
declare @User table([ID] int,[Name] varchar(1),[UserType] int)
insert @User
select 1,'a',1 union all
select 2,'b',1 union all
select 3,'c',2 union all
select 4,'d',2

--> 测试数据：@Expert
declare @Expert table([ID] int,[c1] varchar(1))
insert @Expert
select 1,'a' union all
select 2,'b' union all
select 3,'c' union all
select 4,'d'

--> 测试数据：@Enterprise
declare @Enterprise table([ID] int,[c1] varchar(1))
insert @Enterprise
select 5,'a' union all
select 6,'b' union all
select 7,'c' union all
select 8,'d'

select * from @User a
LEFT JOIN @Expert b ON a.[UserType]=1 AND a.name=b.[c1]
LEFT JOIN @Enterprise c ON a.[UserType]=2 AND a.name=c.[c1] 

/*
ID          Name UserType    ID          c1   ID          c1
----------- ---- ----------- ----------- ---- ----------- ----
1           a    1           1           a    NULL        NULL
2           b    1           2           b    NULL        NULL
3           c    2           NULL        NULL 7           c
4           d    2           NULL        NULL 8           d
*/