mysql - mysql查询的唯一结果

Question

我希望 parent 在此查询中是唯一的：

select 
  *, 
  (select state_name 
   from tbl_states 
   where state_id = tbl_cities.parent_id) as parent 
from 
  tbl_cities 
ORDER BY 
  parent

我试过使用：

select 
  *, 
  DISTINCT (select state_name 
            from tbl_states 
            where state_id = tbl_cities.parent_id) as parent 
from 
  tbl_cities 
ORDER BY 
  parent

但它给出了一个错误。

Answer 1

The order of terms is incorrect. You want

select DISTINCT * ...

Answer 2

从语法上讲，您应该将 DISTINCT 放在子选择中：

select * , 
 (select DISTINCT state_name 
  from tbl_states 
  where state_id = tbl_cities.parent_id) as parent 
from tbl_cities ORDER BY parent

我不明白为什么你会得到多个状态，它们真的是相同的状态重复吗？如果有不同的状态，那么你就有不同的问题。在这种情况下，就语法而言，以下内容将起作用：

select * , 
 (select MAX(state_name) 
  from tbl_states 
  where state_id = tbl_cities.parent_id) as parent 
from tbl_cities ORDER BY parent

但是语义可能不符合您的要求。

Answer 3

像这样重写，并获得符合 ANSI 的额外好处。

select 
  c.*, parent.state_name
from 
  tbl_cities c
  left join (
    select 
      state_id, state_name 
    from 
      tbl_states 
    group by 
      state_id, state_name
  ) as parent 
  on parent.state_id = c.parent_id
order by 
  parent.state_name;