左连接与分组依据

Question

我正在使用 PostgreSQL 9.4。

我有一张桌子workouts。results用户可以为每个创建多个workout，并且一个result有一个score.

给定一个锻炼 ID 列表和两个用户 ID，我想返回每个用户每次锻炼的最佳分数。如果用户没有该锻炼的结果，我想返回填充/空结果。

SELECT "results".*, "workouts".* 
FROM "results" LEFT JOIN "workouts" ON "workouts"."id" = "results"."workout_id" 
WHERE (
  (user_id, workout_id, score) IN 
  (SELECT user_id, workout_id, MAX(score) 
    FROM results WHERE user_id IN (1, 2) AND workout_id IN (1, 2, 3) 
    GROUP BY user_id, workout_id)
) 

在此查询中，左连接充当内连接；如果用户没有得到锻炼结果，我不会得到任何填充。无论存在多少结果，此查询都应始终返回六行。

示例数据：

results
user_id | workout_id | score 
-----------------------------
      1 |          1 |     10
      1 |          3 |     10
      1 |          3 |     15
      2 |          1 |      5

Desired result:

results.user_id | results.workout_id | max(results.score) | workouts.name
-------------------------------------------------------------------------
              1 |                  1 |                 10 | Squat
              1 |                  2 |               null | Bench
              1 |                  3 |                 15 | Deadlift
              2 |                  1 |                  5 | Squat
              2 |                  2 |               null | Bench
              2 |                  3 |               null | Deadlift

Answer 1

distinct on使用or怎么样row_number()？

SELECT DISTINCT ON (r.user_id, r.workout_id) r.*, w.* 
FROM "results" r LEFT JOIN
     "workouts" w
     ON "w."id" = r."workout_id" 
WHERE r.user_id IN (1, 2) AND r.workout_id IN (1, 2, 3) 
ORDER BY r.user_id, r.workout_id, score desc;

等效row_number()的需要一个子查询：

SELECT rw.*
FROM (SELECT r.*, w.*,
             row_number() over (partition by user_id, workout_id order by score desc) as seqnum 
      FROM "results" r LEFT JOIN
           "workouts" w
           ON "w."id" = r."workout_id" 
      WHERE r.user_id IN (1, 2) AND r.workout_id IN (1, 2, 3) 
     ) rw
WHERE seqnum = 1;

您应该比使用 更明智地选择列*。如果列名重复，子查询可能会返回错误。

编辑：

您需要首先生成行，然后生成每行的结果。这是一种基于第二个查询的方法：

SELECT u.user_id, w.workout_id, rw.score, rw.name
FROM (SELECT 1 as user_id UNION ALL SELECT 2) u CROSS JOIN
     (SELECT 1 as workout_id UNION ALL SELECT 2 UNION ALL SELECT 3) w LEFT JOIN
     (SELECT r.*, w.*,
             row_number() over (partition by user_id, workout_id order by score desc) as seqnum 
      FROM "results" r LEFT JOIN
           "workouts" w
           ON "w."id" = r."workout_id" 
      WHERE r.user_id IN (1, 2) AND r.workout_id IN (1, 2, 3) 
     ) rw
     ON rw.user_id = u.user_id and rw.workout_id = w.workout_id and
        rw.seqnum = 1;