同一列的多个 LEFT JOINS

Question

我在 SQL 中，我正在尝试使用多个左连接来填充单个列。我的第一个 LEFT JOIN 如下：

 SELECT a.*, b.Game_Code
  FROM [NCAAF].[dbo].[MasterSheet] a
  LEFT JOIN (SELECT DISTINCT Game_Code, Date, Visit_Team_Code, Home_Team_Code
  from dbo.game2018) b
  ON (a.Date = b.Date and a.[Team Code] = b.Visit_Team_Code and a.[Opponent Code] = b.Home_Team_Code)

所有这一切都来之不易。它创建一个名为 Game_Code 的新列，并填充该列的 1/6，其余列为 NULL。对于其余的空值，我需要执行相同的 Left Join，但切换团队代码的地方除外。这个新的 Left Join 本身想要以下内容：

SELECT a.*, b.Game_Code
  FROM [NCAAF].[dbo].[MasterSheet] a
  LEFT JOIN (SELECT DISTINCT Game_Code, Date, Visit_Team_Code, Home_Team_Code
  from dbo.game2018) b
  ON (a.Date = b.Date and a.[Team Code] = b.Home_Team_Code and a.[Opponent Code] = b.Visit_Team_Code)

虽然我只想对剩余的 NULL 值执行此连接。当我尝试运行两个左连接在一起时，出现错误。我需要这样做的原因是因为每个游戏都列出了两次。一次使用客队代码，然后使用主队代码，然后再次使用主队代码，然后再使用客队代码。我知道这可能令人困惑，但我不想创建多个 Game_Code 列我只需要 1.Please 请参阅下面的示例：两个表在左侧，所需结果在右侧。 First Left Join would satisfy the second row of the desired result and Second Left Join would satisfy the first row, but I can't figure out how to put them together

Answer 1

如果您 LEFT 加入同一个 table 两次，您可以只加入一次并在您的加入中添加一个 OR 并使用两个条件吗？

SELECT a.*, b.Game_Code
  FROM [NCAAF].[dbo].[MasterSheet] a
  LEFT JOIN (SELECT DISTINCT Game_Code, Date, Visit_Team_Code, Home_Team_Code
  from dbo.game2018) b
  ON a.Date = b.Date and 
((a.[Team Code] = b.Visit_Team_Code and a.[Opponent Code] = b.Home_Team_Code)
OR
(a.[Team Code] = b.Home_Team_Code and a.[Opponent Code] = b.Visit_Team_Code))

Answer 2

看起来你只需要 INNER JOIN 而不需要 DISTINCT:

select ms.date, ms.team_code, ms.opponent_code, g.game_code
from dbo.mastersheet ms
join dbo.game2018 g on g.date = ms.date 
                   and g.home_team_code in (ms.team_code, ms.opponent_code)
                   and g.visit_team_code in (ms.team_code, ms.opponent_code)
order by ms.date, ms.team_code, ms.opponent_code;

同一列的多个 LEFT JOINS

Multiple LEFT JOINS for the same column

sql

left-join