使用三张表连接查询并显示缺失记录答案

【问题标题】：Join query using three tables and display missing records使用三张表连接查询并显示缺失记录
【发布时间】：2022-01-23 17:33:03
【问题描述】：

我正在查询的数据库中有三个表：

玩家

Player_id	Player_Name
1	Tom
2	Dick
3	Harry

游戏

Game_id	Game_Name
1	Tennis
2	Rugby
3	Cricket

Games_Players

game_id	player_id	Date_Played
1	2	2021-12-20
1	3	2021-12-20
2	3	2021-12-21
3	2	2021-12-22
3	3	2021-12-22

我想要一个返回玩家 ID、姓名、游戏名称和玩过的日期的查询。如果他们没有玩过游戏——比如 Tom (1) 和 Dick (2)——那么我想要 id、玩家名、游戏名和一个空值。

想要的结果

player_id	player_name	game_name	Date_Played
1	Tom	Tennis	null
1	Tom	Rugby	null
1	Tom	Cricket	null
2	Dick	Tennis	2021-12-20
2	Dick	Rugby	null
2	Dick	Cricket	2021-12-22
3	Harry	Tennis	2021-12-20
3	Harry	Rugby	2021-12-21
3	Harry	Cricket	2021-12-22

我尝试了以下查询，这是我能得到的最接近的查询，并且我尝试了其他联接，但我似乎无法获得所需的所有数据：

SELECT players.player_id, players.player_name, games.game_name, Games_Players.Date_Played,
FROM players
LEFT OUTER JOIN Games_Players
LEFT OUTER JOIN games
ON players.player_id = Games_Players.player_id
AND Games_Players.game_id=games.game_id

它没有返回我需要的所有记录。

【问题讨论】：

标签： sql sql-server

【解决方案1】：

您似乎没有为表Game_Players 添加第一个连接的列。此外，在选择部分的最后一列之后有一个逗号。因此，您的查询应该是：

SELECT players.player_id, 
players.player_name, 
games.game_name, 
Games_Players.Date_Played
FROM players
LEFT OUTER JOIN Games_Players 
ON players.id = Games_Players.player_id
LEFT OUTER JOIN games
ON players.player_id = Games_Players.player_id
AND Games_Players.game_id=games.game_id

【讨论】：

【解决方案2】：

使用cross join 将players 表连接到games 表，然后将结果连接到Games_Players 表

select 
    t1.player_id,
    t1.player_name,
    t1.game_name,
    t2.Date_Played

from
  (select * 
   from players p
   cross join games g) t1
left join Games_Players t2
on t1.Player_id = t2.Player_id and t1.game_id = t2.game_id

db<>fiddle中的演示

【讨论】：