内部连接后的自连接

Question

我正在寻找不同州的城市名称相同。 城市名称和 state 名称位于单独的表（城市和州）中，并且可以通过单独的公共列进行内部连接。

select c1.city, c1.state, c2.city, c2.state 
from cities 
inner join states on cities.commonid = states.commonid

内部加入后，我需要自我加入以执行 function，如下所示

select c1.city, c1.state, c2.city, c2.state
from *joined table* c1 join
     *joined table* c2
     on c1.city = c2.city and c1.state <> c2.state

我想知道如何自我加入一个表，该表是同一查询中另一个加入的结果 output 将是这样的

+----------+-------+--------+--------+
| city1    | state1|city2   |state2  |
+----------+-------+--------+--------+
| x        | melb  | x      | syd    |
| y        | bris  | y      | ACT    |
+----------+-------+--------+--------+

Answer 1

我假设表cities有一个像state_id这样的列，它引用表states中的列state_id （将名称更改为列的实际名称）。
首先对有条件的cities进行自加入：

c1.city = c2.city AND c1.state_id < c2.state_id

<运算符确保每对城市只返回一次。
然后加入 2 个states副本，因为它们每个都将用于获取 2 个城市中每个城市的 state 的名称：

SELECT c1.city city1, s1.state state1,
       c2.city city2, s2.state state2
FROM cities c1 
INNER JOIN cities c2 ON c1.city = c2.city AND c1.state_id < c2.state_id
INNER JOIN states s1 ON s1.state_id = c1.state_id
INNER JOIN states s2 ON s2.state_id = c2.state_id
ORDER BY city1

Answer 2

您可以对内部连接查询执行 select 并为其指定别名。

然后它会变成像下面这样......

Select c.city,c.state from
(Select City,state from cities inner join states where cities.id = states.id) as c

---

现在为 c 进行自我连接。

Answer 3

我将对拥有多个 state 的所有城市进行预查询。 然后加入 states 表，看看他们遇到了哪些状态。

    select
          Duplicates.city,
          s.state
       from
          ( select c1.city
               from cities c1
               group by c1.city
               having count(*) > 1 ) Duplicates
             JOIN cities c2
                on Duplicates.city = c2.city
                JOIN states s
                   on c2.commonid = s.commonid
      order by
         Duplicates.city,
         s.state

通过不尝试仅对两个城市/州进行交叉表，您将获得一个列表。 如果一个城市名称存在于 5 个州，您打算如何显示它。 这样你会看到所有的字母。

Answer 4

我会建议一个CTE：

with cs as (
      select c.name as city_name, s.name as state_name
      from cities c join
           states s
           on c.commonid = s.commonid
     )
select cs1.*, cs2.*
from cs cs1 join
     cs cs2
     on cs1.name = cs2.name and cs1.state <> cs2.state;