Question

给定一个具有值（1,1）（1,4），（4,1）（2,2）的表（c1，c2），什么查询将允许我只输出c2 = 1的结果并且表中只有一个条目？在这种情况下，它只是（4,1），因为有两个条目，其中c1 = 1。欢呼声。

Answer 1

嗯..有很多方法。这是一个：

select c1, c2
from my_table t1
where c2 = 1
  and 1 = (select count(*) from my_table t2 where t2.c1 = t1.c1)

其他一些方式：

select t1.c1, t1.c2
from my_table t1
join my_table t2 using (c1)
where t1.c2 = 1
group by t1.c1, t1.c2
having count(*) = 1;

select c1, c2
from my_table t1
where c2 = 1
  and not exists (
      select *
      from from my_table t2
      where t2.c1 =  t1.c1
        and t2.c2 <> t1.c2
  );

select t1.c1, t1.c2
from my_table t1
left join my_table t2
  on  t2.c1 =  t1.c1
  and t2.c2 <> t1.c2
where t1.c2 = 1
  and t2.c1 is null;

最后两个查询要求您的表格中没有重复项。

SQL查询类似于distinct

1 个答案: