从两个表中选择所有列，按表1中的所有列和表2中的特定列分组

Question

我的表结构如下

    TblMemberInfo          |         TblCarInfo
 MemberID   Name           |         Id     MemberId     CarNumber
    1       Sandeep        |         1         2            1234
    2       Vishal         |         2         1            1111
    3       John           |         3         4            2458
    4       Kevin          |         4         2            1296
    5       Devid          |         5         4            7878
                           |         6         3            4859

我需要从TblMemberInfo，TblCarInfo查询全选，其中Count（MemberId）= 1

      MemberId    Name        CarNumber
        1        Sandeep        1111
        3        John           4859

Answer 1

这是一种方法：

select mi.MemberID, mi.Name, min(CarNumber) as CarNumber
from TblMemberInfo mi join
     TblCarInfo ci
     on mi.MemberID = ci.MemberID
group by mi.MemberID, mi.Name
having count(*) = 1;

之所以可行，是因为组中只有一行， min()返回正确的值。

而且not exists替代方法：

select mi.MemberID, mi.Name, ci.CarNumber
from TblMemberInfo mi join
     TblCarInfo ci
     on mi.MemberID = ci.MemberID
where not exists (select 1
                  from TblCarInfo ci2
                  where ci2.MemberID = ci.MemberID and ci2.id <> ci.id
                 );

Answer 2

还有更多选择！

select mi.MemberId, mi.Name, ci.CarNumber
from TblMemberInfo mi 
join TblCarInfo ci on 
   mi.MemberId = ci.MemberId
group by mi.MemberId, mi.Name, ci.CarNumber
having min(ci.Id) = max(ci.Id)

如果您还需要引入许多其他列，则使用子查询来检索单个MemberId's是一个好主意。

select mi.MemberId, mi.Name, ci.CarNumber
from TblMemberInfo mi   
join TblCarInfo ci on 
   mi.MemberId = ci.MemberId
where mi.MemberId in
(
   select MemberId     
   from TblCarInfo 
   group by MemberId
   having count(*) = 1
)

Answer 3

你会怎么办授予这项任务？ 可能：首先查找唯一的TblCarInfo成员条目，然后查找成员名称。 因此，告诉DBMS完全这样做：

select m.memberid, m.name, c.carnumber
from
(
  select memberid, min(carnumber) as carnumber
  from tblcarinfo
  group by memberid
  having count(*) = 1
) c
join tblmemberinfo m on m.memberid = c.memberid;

或相同的方法，但在select子句中使用子查询而不是联接：

select
  c.memberid, 
  (select m.name from tblmemberinfo m where m.memberid = c.memberid) as name,
  c.carnumber
from
(
  select memberid, min(carnumber) as carnumber
  from tblcarinfo
  group by memberid
  having count(*) = 1
) c;