如何在SQL中聯接這些表

Question

我有以下表格：

   match

 id   |   rival_team
----------------------
|  1  |   chelsea fc
|  2  |   real madrid


   player

 ID   |   name    |   last_name  |
---------------------------------
|  1  |    John   |      Doe     |
|  2  |   Peter   |   Williams   |


   called_up_players  

 match_id   |   player_id  |  substitution_id  |    
---------------------------|-------------------|
|     1     |      1       |         1         |
|     1     |      2       |        NULL       |


   substitution

|     id    |   match_id   | substitute_player_id | replaced_player_id |
---------------------------|----------------------|--------------------|
|     1     |      1       |           1          |          2         |

我有以下SQL語句

SELECT called_up_players.match_id, match.rival_team, player.name, 
           player.last_name, substitution.id
FROM called_up_players, substitution, player, match
WHERE called_up_players.substitution_id = substitution.substitute_player_id
         AND player.id = called_up_players.substitution_id;

和以下輸出：

match_id| rival_team |   name   | last_name | substitution_id
--------+------------+----------+-----------+-----------
      1 | chelsea fc |   John   |    Doe    |         1
      1 | real madrid|   John   |    Doe    |         1
(2 rows)

但是，我希望輸出像

match_id| rival_team |   name   | last_name | substitution_id
--------+------------+----------+-----=-----+----------------
      1 | chelsea fc |   John   |    Doe    |         1
      2 | real madrid|   John   |    Doe    |        NULL

顯示所有被召集到John Doe名單上的比賽。 無論球員參與換人，我都希望有名稱和替換ID的列

我想我可以使用JOINS來實現這一點，但是我不知道如何聯接表，因此可以得到與上述輸出類似的東西。 我嘗試了很多聲明我出錯了。

Answer 1

您要加入匹配項而沒有加入條件，因此這將成為交叉聯接，而不是內部聯接。

這是為什么將不同的聯接語法視為最佳實踐的一個示例：

FROM called_up_players  
INNER JOIN substitution ON called_up_players.substitution_id = substitution.substitute_player_id  
 INNER JOIN player ON player.id = called_up_players.substitution_id  
 INNER JOIN match ON ???

Answer 2

多虧了Learning2Code和他的回答，我對聯接有了更多的了解，最后，我想出了一個獲得所需輸出的答案。

我不使用INNER JOIN，而是使用LEFT JOIN，以便從左側的表中檢索所有記錄，如果右側的表中沒有數據，則該字段為空

SELECT match_id, rival_team, name, last_name, substitution_id        
FROM match  
LEFT JOIN called_up_players ON called_up_players.match_id = match.id
LEFT JOIN player ON player.id = called_up_player.player_id
LEFT JOIN substitution ON called_up_players.substitution_id = substitution.substitution_id
WHERE player.name = 'John' AND player.last_name = 'Doe';

輸出：

   partido_id | contrario  |  nombre    | apellido  | cambio_id
 -------------+------------+------------+-----------+------------
            1 | chelsea fc | John       | Doe       |         1
            2 | real madrid| John       | Doe       |
 (2 rows)