Sql 如果A中存在记录，则返回记录else如果B中存在记录，则返回记录else null

我试图用SQL来表达这个想法

If record exist in A then return record else if record exist in B return record ELSE NULL

示例：选择队长（如果有），否则选择队伍中年龄最大的球员

WITH Captains_CTE (TeamName, PlayerName)
AS (
    SELECT TeamName, PlayerName FROM TeamPlayer WHERE IsCaptain = 1
), 
OldestPlayer_CTE (TeamName, PlayerName)
AS (
    SELECT TeamName, PlayerName FROM TeamPlayer AS tp
    INNER JOIN ( 
        SELECT TeamName, MAX(PlayerAge) AS MaxAge
        FROM TeamPlayer 
        GROUP BY TeamName
    ) AS old ON old.TeamName = tp.TeamName AND old.MaxAge = tp.PlayerAge

)
SELECT  CASE 
            WHEN Captains_CTE.TeamName IS NULL THEN OldestPlayer_CTE.TeamName
            ELSE Captains_CTE.TeamName 
        END AS TeamName,
        CASE 
            WHEN Captains_CTE.PlayerName IS NULL THEN OldestPlayer_CTE.PlayerName
            ELSE Captains_CTE.PlayerName 
        END AS PlayerName
FROM Captains_CTE
FULL OUTER JOIN OldestPlayer_CTE ON Captains_CTE.TeamName = OldestPlayer_CTE.TeamName

---

我可以通过在a和B上进行外部联接，然后为我希望返回的每个列构建一个CASE语句来实现这一点，但必须有更好的方法。（是的，我的示例查询对年龄的唯一性做出了一些可怕的假设）

使用

不存在
：

select a.*
from a
union all
select b.*
from b
where not exists (select 1 from a where a.? = b.?);  -- "?" is for the column that specifies whether the record exists in A

您可以使用
APPLY
执行此操作：

SELECT t.TeamName, coalsesce(captain.PlayerName,'') As Captain
FROM Teams t
OUTER APPLY (
   SELECT TOP 1 PlayerName
   FROM TeamPlayers tp
   WHERE tp.TeamID = t.TeamID
   ORDER BY CASE WHEN tp.IsCaptain = 1 THEN 1 ELSE 2 END,
        PlayerAge DESC
) captain

如果没有
Teams
表（如果模式有任何意义，应该有，但您永远不知道），您可以派生一个：

SELECT t.TeamName, coalsesce(captain.PlayerName,'') As Captain
FROM (SELECT DISTINCT TeamName FROM TeamPlayers) t
OUTER APPLY (
   SELECT TOP 1 PlayerName
   FROM TeamPlayers tp
   WHERE tp.TeamName = t.TeamName 
   ORDER BY CASE WHEN tp.IsCaptain = 1 THEN 1 ELSE 2 END,
        PlayerAge DESC
) captain

我不知道代码和这个问题有什么关系。例如，代码中没有任何内容被称为
a
或
b
。您可以使用
COALESCE（）
来简化
案例，但这只是一个旁注。我的实际答案如下。@GordonLinoff队长代码只是我想出的一个例子，而不是发布我真正可怕的50行ERP查询。