How to get the opposite of a join? - mysql

Question

I am trying to get the rows that don't exist in one table where one table called schedules (match_week, player_home_id, player_away_id) and the other table called match (match_week, Winner_id, Defeated_id) are joined. The players look at their schedule and play a match. I am trying to get a list of the scheduled matches that do not exist in the match table. The IDs in the match table can be in either column Winner_id or Defeated_id.
I have reviewed a number of Stack Exchange examples, but most use "IS NULL" and I don't have null values. I have used a Join that does give the output of the matches played. I would like the matches that have not been played.
CSV - wp_schedule_test
+----+------------+--------------+--------------+-----------------+-----------------+
| ID | match_week | home_player1 | away_player1 | player1_home_id | player1_away_id |
+----+------------+--------------+--------------+-----------------+-----------------+
| 1 | WEEK 1 | James Rives | Dale Hemme | 164 | 169 |
| 2 | WEEK 1 | John Head | David Foster | 81 | 175 |
| 3 | WEEK 1 | John Dalton | Eric Simmons | 82 | 23 |
| 4 | WEEK 2 | John Head | James Rives | 81 | 164 |
| 5 | WEEK 2 | Dale Hemme | John Dalton | 169 | 82 |
| 6 | WEEK 2 | David Foster | Eric Simmons | 175 | 23 |
| 7 | WEEK 3 | John Dalton | James Rives | 82 | 164 |
| 8 | WEEK 3 | John Head | Eric Simmons | 81 | 23 |
| 9 | WEEK 3 | Dale Hemme | David Foster | 169 | 175 |
| 10 | WEEK 4 | Eric Simmons | James Rives | 23 | 164 |
| 11 | WEEK 4 | David Foster | John Dalton | 175 | 82 |
| 12 | WEEK 4 | Dale Hemme | John Head | 169 | 81 |
+----+------------+--------------+--------------+-----------------+-----------------+
CSV - wp_match_scores_test
+----+------------+------------+------------+
| ID | match_week | player1_id | player2_id |
+----+------------+------------+------------+
| 5 | WEEK 1 | 82 | 23 |
| 20 | WEEK 1 | 164 | 169 |
| 21 | WEEK 2 | 164 | 81 |
| 25 | WEEK 2 | 82 | 169 |
| 61 | WEEK 3 | 175 | 169 |
| 62 | WEEK 4 | 175 | 82 |
| 69 | WEEK 2 | 175 | 23 |
| 85 | WEEK 3 | 164 | 82 |
| 86 | WEEK 4 | 164 | 23 |
+----+------------+------------+------------+
The output from the mysql query are the matches that have been played. I am trying to figure out how to list the matches that have not been played from the table Schedule.
CSV - MySQL Output
+------------+------------+------------+
| match_week | player1_id | player2_id |
+------------+------------+------------+
| WEEK 1 | 164 | 169 |
| WEEK 1 | 82 | 23 |
| WEEK 2 | 164 | 81 |
| WEEK 2 | 82 | 169 |
| WEEK 2 | 175 | 23 |
| WEEK 3 | 175 | 169 |
| WEEK 3 | 164 | 82 |
| WEEK 4 | 175 | 82 |
| WEEK 4 | 164 | 23 |
+------------+------------+------------+
MYSQL
select DISTINCT ms.match_week, ms.player1_id , ms.player2_id FROM
wp_match_scores_test ms
JOIN wp_schedules_test s
ON (s.player1_home_id = ms.player1_id or s.player1_away_id =
ms.player2_id)
Order by ms.match_week
The expected output is:
CSV - Desired Output
+------------+----------------+----------------+
| match_week | player_home_id | player_away_id |
+------------+----------------+----------------+
| WEEK 1 | 81 | 175 |
| WEEK 3 | 81 | 23 |
| WEEK 4 | 169 | 81 |
+------------+----------------+----------------+
The added code I would like to use is
SELECT s.*
FROM wp_schedules_test s
WHERE NOT EXISTS
(select DISTINCT ms.match_week, ms.player1_id , ms.player2_id FROM
wp_match_scores_test ms
JOIN wp_schedules_test s
ON (s.player1_home_id = ms.player1_id or s.player1_away_id =
ms.player2_id)
Order by ms.match_week)
Unfortunately, the output yields "No Rows"

Answer 1

You can use a LEFT JOIN to achieve the desired results, joining the two tables on matching player ids (noting that player id values in wp_match_scores_test can correspond to either player1_home_id or player1_away_id in wp_schedules_test). If there is no match, the result table will have NULL values from the wp_match_scores_test table values, and you can use that to select the matches which have not been played:
SELECT sch.*
FROM wp_schedule_test sch
LEFT JOIN wp_match_scores_test ms
ON (ms.player1_id = sch.player1_home_id
OR ms.player2_id = sch.player1_home_id)
AND (ms.player1_id = sch.player1_away_id
OR ms.player2_id = sch.player1_away_id)
WHERE ms.ID IS NULL
Output:
ID match_week home_player1 away_player1 player1_home_id player1_away_id
2 Week 1 John Head David Foster 81 175
8 Week 3 John Head Eric Simmons 81 23
12 Week 4 Dale Hemme John Head 169 81
Note that you can also use a NOT EXISTS query, using the same condition as I used in the JOIN:
SELECT sch.*
FROM wp_schedule_test sch
WHERE NOT EXISTS (SELECT *
FROM wp_match_scores_test ms
WHERE (ms.player1_id = sch.player1_home_id
OR ms.player2_id = sch.player1_home_id)
AND (ms.player1_id = sch.player1_away_id
OR ms.player2_id = sch.player1_away_id))
The output of this query is the same. Note though that conditions in the WHERE clause have to be evaluated for every row in the result set and that will generally make this query less efficient than the LEFT JOIN equivalent.
Demo on dbfiddle