Question

对于下表（我用**来表示主键的部分）

Team (*tid*, name)
Game (*gid*, tid, name, year)
Member (*mid*, name)
MemberOf (*mid*, *tid*, startyear, endyear, position)

我需要显示每个前锋的历史（位置）如果startyear为NULL，请使用该球员最早的比赛中该球员的年份结果：前锋名称，球队名称，开始年份，结束年份按Strikers名称排序，然后是开始年份

虽然我有一个解决方案，但我认为它不是很有效率执行计划推断节点成本为28。希望改进建议和解释吗？

Answer 1

您可以使用left outer join和coalesce通过一个查询执行此操作（如果我的逻辑正确）：

SELECT Striker_Name, Team_Name, coalesce(MO.STARTYEAR, g.start_year), MO.ENDYEAR
FROM (SELECT NAME AS Striker_Name, MID
      FROM MEMBER
      ) ME JOIN
      (SELECT MID, TID, STARTYEAR AS Start_Year, ENDYEAR AS End_Year
       FROM MEMBEROF
       WHERE POSITION = 'striker'
      ) MO ON ME.MID = MO.MID left outer JOIN
      (SELECT NAME AS Team_Name, TID
       FROM TEAM
      ) T
      ON MO.TID = T.TID left outer JOIN
      (SELECT GID, MIN(GAME.YEAR) AS Start_Year FROM GAME GROUP BY TID
      ) G
      ON G.TID = MO.TID
ORDER BY Striker_Name, Start_Year

你似乎从最早的比赛开始填补了这一年。只需进行连接并在select中决定使用哪一个。

SQL Query，试图提高效率（Oracle DBMS）

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