内部联接后的自联接

如何解决内部联接后的自联接

我正在查找不同州中哪些城市具有相同的名称。城市名称和州名称位于单独的表（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，我需要自加入以执行如下功能

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

我想知道如何自连接一个表，该表是同一查询中另一个连接的结果 输出将是这样的

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

解决方法

我假设表 cities 有一个类似于 state_id 的列，它引用表 state_id 中的列 states（将名称更改为列的实际名称).
首先使用条件为 cities 执行自联接：

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 states 的 2 个副本，因为每个副本都将用于获取 2 个城市中每个城市的州名：

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

,

您可以选择内部连接查询并为其指定别名。

然后就会变成下面这样...

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 创建一个自连接。

,

我将对拥有多个州的所有城市执行预查询。然后加入states表，看看它们遇到了哪些状态。

    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

不要尝试仅对两个城市/州进行交叉表，您将得到一个列表。如果 5 个州存在一个城市名称，您打算如何显示它。这样你就会看到所有的字母顺序。

,

我建议 CTE：

#See https://aka.ms/containerfastmode to understand how Visual Studio uses this Dockerfile to build your images for faster debugging.

FROM mcr.microsoft.com/dotnet/aspnet:5.0 AS base
WORKDIR /app
EXPOSE 5000
EXPOSE 5001

FROM mcr.microsoft.com/dotnet/sdk:5.0 AS build
WORKDIR /src
COPY ["src/Skybot.Web/Skybot.Web.csproj","src/Skybot.Web/"]
COPY ["src/Skybot.Application/Skybot.Application.csproj","src/Skybot.Application/"]
COPY ["src/Skybot.Domain/Skybot.Domain.csproj","src/Skybot.Domain/"]
COPY ["src/Skybot.Infrastructure/Skybot.Infrastructure.csproj","src/Skybot.Infrastructure/"]
RUN dotnet restore "src/Skybot.Web/Skybot.Web.csproj"
COPY . .
WORKDIR "/src/src/Skybot.Web"
RUN dotnet build "Skybot.Web.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "Skybot.Web.csproj" -c Release -o /app/publish

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet","Skybot.Web.dll"]