无法通过左连接将两个键连接到一个主键

Question

SELECT teams.teamname AS Home, 
       teams.teamname AS Away 
FROM   game 
       LEFT JOIN teams 
              ON game.awayteam = teams.teamid 
       LEFT JOIN teams AS T 
              ON game.hometeam = T.teamid

我的表格如下

Game                                     
-GameId(Pk,int),
-HomeTeamId (fk,int),   
-AwayTeamId(fk,int)                     

Teams
-TeamId(Pk,int),
-TeamName(Varchar)

我正在尝试在 query.When 中查看主队和客队名称我加入表格我的预期结果是主队名称和客队名称，但是在两个表格中只有主队名称被列出。

Home Team|Spikes

Away Team|Spikes

什么时候应该

Home Team|Spikes

Away Team|Raptors

Answer 1

你需要给两个加入一个别名。您正在两次选择第一次连接的结果。

例如

SELECT
  GameId,
  home.TeamName AS Home,
  away.TeamName AS Away
FROM Game
  LEFT JOIN Teams AS away ON AwayTeamId = away.TeamId
  LEFT JOIN Teams AS home ON HomeTeamId = home.TeamId;

Answer 2

如果您将重命名列推送到连接中的 table 表达式，您可以使用 NATURAL JOIN 并避免完全使用范围变量。另外，我不相信你真的需要外连接：

SELECT Home, Away
  FROM game
       NATURAL JOIN
       ( SELECT teamid AS hometeam
              teamname AS home
         FROM teams ) Home
       NATURAL JOIN
       ( SELECT teamid AS awayteam
              teamname AS Away
         FROM teams ) away;

无法通过左连接将两个键连接到一个主键

Unable to Join two keys to one Primary key with left join

alias

join