如何在SQL中联接这些表

Question

I have the following tables:

   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         |

I have the following SQL statement

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;

and the following output:

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

However, I want the output to be like

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

to display all matches where John Doe was called up to the roster. Regardless the player was involved in a substitution, I would like to have the columns for the name and substitution_id

I think I can achieve this using JOINS , but I don't know how to join tables so I can have something similar to the above output. I tried many statement I got errors.

Answer 1

You are joining to match without a join condition, so this becomes a cross join not an inner join.

This is one example of why a different join syntax is considered best practice:

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

thanks to Learning2Code and his answer I understood a bit more about joins and finally, I came up with an answer to get the output I wanted.

Instead of using INNER JOIN, I use LEFT JOIN so that, all records are retrieved from the table on the left and if there is no data from the table on the right, the field is just empty

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';

Output:

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