Question

我需要从PostgreSQL表中查询预定义的技能等级集。考虑以下具有用户1,2和3的演示表，每个用户具有不同的技能和技能等级：

╔════════╦═══════════╦═══════╗
║ userId ║  skillId  ║ votes ║
╠════════╬═══════════╬═══════╣
║      1 ║ marketing ║    50 ║
║      2 ║ hacking   ║    51 ║
║      1 ║ business  ║    59 ║
║      2 ║ business  ║     8 ║
║      1 ║ hacking   ║    32 ║
║      3 ║ talking   ║    67 ║
║      3 ║ business  ║    34 ║
║      3 ║ marketing ║    24 ║
╚════════╩═══════════╩═══════╝

现在我想根据加权业务+黑客对其进行排名：

SELECT * FROM
  (SELECT
    b."userId",
    MAX(CASE WHEN b."skillId"='business' THEN b.votes ELSE 0 END) AS "businessVotes",
    MAX(CASE WHEN h."skillId"='hacking' THEN h.votes ELSE 0 END) AS "hackingVotes"
    FROM "Skill" b, "Skill" h
    WHERE b."userId"=h."userId"
    AND (
      (b."skillId"='business' AND h."skillId"='hacking') OR
      (b."skillId"=h."skillId" AND (b."skillId"='business' OR b."skillId"='hacking'))
    )
    GROUP BY b."userId") AS query
  ORDER BY "businessVotes"*0.2+"hackingVotes"*0.8 DESC

正如预期的那样，我得到以下结果：

╔════════╦═══════════════╦══════════════╗
║ userId ║ businessVotes ║ hackingVotes ║
╠════════╬═══════════════╬══════════════╣
║      2 ║             8 ║           51 ║
║      1 ║            59 ║           32 ║
║      3 ║            34 ║            0 ║
╚════════╩═══════════════╩══════════════╝

但在我看来，这个查询要求PostgreSQL进行比必要更多的计算，因为错综复杂地处理了用户没有黑客但业务技能或其他方式的情况。如果没有这种情况（即商业和黑客技能都需要进入排名），查询可能会容易得多：

SELECT *
  FROM "Skill" t1, "Skill" t2
  WHERE t1."userId"=t2."userId"
  AND (t1."skillId"='business'
  AND t2."skillId"='hacking')
  ORDER BY t1."votes"*0.2 + t2."votes"*0.8 DESC;

是因为我的推理错误，一个不利的表结构还是因为它的方式而且没有更好的选择？

Answer 1

您可以使用聚合查询执行此操作：

select s.userId,
       sum((case when s.skillId = 'business' then votes else 0 end)*0.2 +
           (case when s.skillId = 'hacking' then votes else 0 end)*0.8
          ) as weightedpoints
from skill s
group by userId
order by weightedpoints desc

SQL查询表的技巧

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