从一个表中选择所有条目，从另一个“日志记录”表中选择 LATEST 条目

Question

我已尝试为我的问题准备 SQL Fiddle -

在多人文字游戏中，活动游戏存储在表中words_games：

CREATE TABLE words_games (
        gid SERIAL PRIMARY KEY,              /* game id */
        created timestamptz NOT NULL,

        player1 integer REFERENCES words_users(uid) ON DELETE CASCADE NOT NULL,
        player2 integer REFERENCES words_users(uid) ON DELETE CASCADE,

        played1 timestamptz,
        played2 timestamptz,

        score1 integer NOT NULL CHECK(score1 >= 0),
        score2 integer NOT NULL CHECK(score2 >= 0),

        hand1 varchar[7] NOT NULL,
        hand2 varchar[7] NOT NULL,
        pile  varchar[116] NOT NULL,

        letters varchar[15][15] NOT NULL,
        values integer[15][15] NOT NULL,
        bid integer NOT NULL REFERENCES words_boards ON DELETE CASCADE
);

而且很容易选择所有游戏，例如，ID为1的玩家参与的游戏：

SELECT * FROM words_games WHERE player1 = 1 OR player2 = 1;

但现在我还添加了一个表格words_moves，它作为玩家操作的日志记录：

CREATE TYPE words_action AS ENUM ('play', 'skip', 'swap', 'resign');

CREATE TABLE words_moves (
        mid SERIAL PRIMARY KEY,             /* move id */
        action words_action NOT NULL,
        gid integer NOT NULL REFERENCES words_games ON DELETE CASCADE,
        uid integer NOT NULL REFERENCES words_users ON DELETE CASCADE,
        played timestamptz NOT NULL,
        tiles jsonb NULL,
        score integer NULL CHECK(score > 0) /* score awarded in that move */
);

现在，当用户连接到我的游戏服务器时，我不仅要向她发送所有活动游戏，还要向她发送每个游戏的最新动作（最高 mid）。

请问如何在一个查询中运行这样的连接（或 CTE）？

我尝试了以下 INNER JOIN，但它返回 所有 招式，而我只需要每场比赛中的 最新 招式：

SELECT
    g.gid,
    EXTRACT(EPOCH FROM g.created)::int AS created,
    g.player1,
    COALESCE(g.player2, 0) AS player2,
    COALESCE(EXTRACT(EPOCH FROM g.played1)::int, 0) AS played1,
    COALESCE(EXTRACT(EPOCH FROM g.played2)::int, 0) AS played2,
    ARRAY_TO_STRING(g.hand1, '') AS hand1,
    ARRAY_TO_STRING(g.hand2, '') AS hand2,
    -- g.letters,
    -- g.values,
    m.action,
    m.tiles                                                                                                                                                                   
FROM words_games g INNER JOIN words_moves m                                                                                                                                          
    ON g.gid = m.gid                                                                                                                                                                  
    AND ( g.player1 = m.uid OR g.player2 = m.uid )                                                                                                                                     
    AND ( g.player1 = 1 OR g.player2 = 1 )                                                                                                                                              
ORDER BY g.gid;


     gid |  created   | player1 | player2 |  played1   |  played2   |  hand1  |  hand2  | action |                                                                                                                                                                          tiles                                                                                                                                                                          
    -----+------------+---------+---------+------------+------------+---------+---------+--------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
       1 | 1471794994 |       1 |       2 | 1471868012 | 1471810486 | ПЕАЯСАС | ЖИОБАЯС | play   | [{"col": 7, "row": 10, "value": 1, "letter": "Н"}, {"col": 7, "row": 8, "value": 2, "letter": "К"}, {"col": 7, "row": 9, "value": 1, "letter": "И"}, {"col": 7, "row": 7, "value": 2, "letter": "С"}]
       1 | 1471794994 |       1 |       2 | 1471868012 | 1471810486 | ПЕАЯСАС | ЖИОБАЯС | play   | [{"col": 7, "row": 14, "value": 2, "letter": "К"}, {"col": 7, "row": 13, "value": 1, "letter": "Н"}, {"col": 7, "row": 11, "value": 3, "letter": "У"}, {"col": 7, "row": 12, "value": 2, "letter": "П"}]
       1 | 1471794994 |       1 |       2 | 1471868012 | 1471810486 | ПЕАЯСАС | ЖИОБАЯС | play   | [{"col": 6, "row": 2, "value": 2, "letter": "П"}, {"col": 6, "row": 3, "value": 1, "letter": "О"}, {"col": 6, "row": 4, "value": 1, "letter": "Е"}, {"col": 6, "row": 5, "value": 5, "letter": "Ж"}, {"col": 6, "row": 6, "value": 5, "letter": "Ы"}, {"col": 6, "row": 7, "value": 2, "letter": "П"}, {"col": 6, "row": 8, "value": 5, "letter": "Ы"}]
       2 | 1471795037 |       1 |       2 | 1471806484 | 1471865696 | КЙВГКСМ | ЯРХЖИМН | swap   | "А"
       2 | 1471795037 |       1 |       2 | 1471806484 | 1471865696 | КЙВГКСМ | ЯРХЖИМН | play   | [{"col": 7, "row": 10, "value": 5, "letter": "Ы"}, {"col": 7, "row": 9, "value": 2, "letter": "Д"}, {"col": 7, "row": 8, "value": 1, "letter": "А"}, {"col": 7, "row": 7, "value": 2, "letter": "Л"}]
    (5 rows)

更新：

实际上我需要一个 LEFT JOIN，因为有些游戏可能还没有任何玩家动作...

Answer 1

好的，让我们构建 sql。首先，我们需要弄清楚所有游戏的最新动作。有很多方法可以做到这一点，但让我们试试这个：

SELECT *
FROM words_moves wm1
WHERE
  played = (SELECT max(played)
            FROM words_moves wm2
            WHERE wm1.gid = wm2.gid);

这不是最快的方法，但它是一种更容易理解的方法——从时间戳最新的words_moves 获取每一个动作。

现在我们有了它，我们可以用它构建一个查询来获取游戏和动作：

WITH last_moves AS (
  SELECT *
  FROM words_moves wm1
  WHERE
    played = (SELECT max(played)
              FROM words_moves wm2
              WHERE wm1.gid = wm2.gid))
SELECT *
FROM words_games wg
  LEFT JOIN last_moves lm
    ON (wg.gid = lm.gid)
WHERE
  player1 = 1 OR
  player2 = 1;

如果您不熟悉，那里的WITH 表示common table expression，这是一种非常方便的子查询。除其他外，这意味着如果您最终使用不同的方法来获取每场比赛的最新动作（this question 有很多可供尝试的替代方法），那么很容易换掉而不会有太多麻烦。

希望有帮助！

Answer 2

SELECT g.gid
    , EXTRACT(EPOCH FROM g.created)::int AS created
    , g.player1
    , COALESCE(g.player2, 0) AS player2
    , COALESCE(EXTRACT(EPOCH FROM g.played1)::int, 0) AS played1
    , COALESCE(EXTRACT(EPOCH FROM g.played2)::int, 0) AS played2
    , ARRAY_TO_STRING(g.hand1, '') AS hand1
    , ARRAY_TO_STRING(g.hand2, '') AS hand2
    , m.action
    , m.tiles 
FROM words_games g  
LEFT JOIN words_moves m
    ON g.gid = m.gid
        -- this is redundant: m.gid is a FK
        -- AND (g.player1 = m.uid OR g.player2 = m.uid)
    AND NOT EXISTS ( -- suppress all-but-the-last
        SELECT * FROM words_moves nx
        WHERE nx.gid = g.gid -- Same game
          -- AND nx.mid > m.mid   -- but a higher moveid
                                  -- (assuming ascending move_ids)
          -- or: you could use m.played, if that is ascending
        AND nx.played > m.played
        )
WHERE (g.player1 = 1 OR g.player2 = 1)
ORDER BY g.gid;