Question

我希望父查询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

但它会出错。

Answer 1

条款的顺序不正确。你想要

select DISTINCT * ...

Answer 2

从语法上讲，你应该把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州，他们真的是同一个州重复吗？如果有不同的状态，那么你有一个不同的问题。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

但语义可能对您的要求不合适。

Answer 3

像这样重写，并获得ANSI合规性的额外好处。

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;

来自mysql查询的独特结果

3 个答案: