选择1到n，比较Mysql中每一个的倒数第二行

Question

I would like to extract the doctors for whom their penultimate patient in reverse creation order was created in 2016

DOCTORS

| id |  name  |
|  1 |  Ryan  |
|  2 |  Pete  |
|  3 |  Anna  |
|  4 |  Harry |

PATIENTS

| id |  name  | surgeon_id | created_at |
|  1 | Gloria |     1      | 2016-05-01 |
|  2 | Bob    |     1      | 2016-06-21 |
|  3 | Alex   |     2      | 2015-05-01 |
|  4 | Jim    |     2      | 2016-05-01 |
|  3 | Kay    |     3      | 2016-05-01 |
|  5 | Kim    |     4      | 2016-05-01 |
|  6 | Joe    |     4      | 2017-01-03 |

So the result must be Ryan(1) and Harry(4) because of:

|  1 | Gloria |     1      | 2016-05-01 |
|  5 | Kim    |     4      | 2016-05-01 |

Answer 1

DEMO

First you use variable to assign a position to each patient.

SELECT `id`, `name`, `surgeon_id`, `created_at`,
       @pos := IF(@surgeon_id = surgeon_id, 
                  @pos + 1, 
                  IF(@surgeon_id := surgeon_id, 1, 1) 
                 ) as rn
FROM Table1
CROSS JOIN (SELECT @pos := 0, @surgeon_id :=0 ) as parameters
ORDER BY `surgeon_id`, `created_at` DESC

Then use that as a subquery to get the second to last patient and test the year.

SELECT `id`, `name`, `surgeon_id`, `created_at`
FROM (
        SELECT `id`, `name`, `surgeon_id`, `created_at`,
               @pos := IF(@surgeon_id = surgeon_id, 
                          @pos + 1, 
                          IF(@surgeon_id := surgeon_id, 1, 1) 
                         ) as rn
        FROM Table1
        CROSS JOIN (SELECT @pos := 0, @surgeon_id :=0 ) as parameters
        ORDER BY `surgeon_id`, `created_at` DESC
     ) T
WHERE T.rn = 2
  AND YEAR(`created_at`) = 2016

Finally Join back to Doctors to get the name

SELECT Doctors.`id`, Doctors.`name`
FROM (
        SELECT `id`, `name`, `surgeon_id`, `created_at`,
               @pos := IF(@surgeon_id = surgeon_id, 
                          @pos + 1, 
                          IF(@surgeon_id := surgeon_id, 1, 1) 
                         ) as rn
        FROM Patients
        CROSS JOIN (SELECT @pos := 0, @surgeon_id :=0 ) as parameters
        ORDER BY `surgeon_id`, `created_at` DESC
     ) T
JOIN Doctors
  ON T.`surgeon_id` = Doctors.`id`
WHERE T.rn = 2
  AND YEAR(`created_at`) = 2016; 

OUPUT: