Question

我需要执行一个相对容易解释但是（考虑到我的技能有限）难以编写SQL查询。

假设我们有一个类似于这个的表：

 exam_no | name | surname | result | date
---------+------+---------+--------+------------
 1       | John | Doe     | PASS   | 2012-01-01
 1       | Ryan | Smith   | FAIL   | 2012-01-02 <--
 1       | Ann  | Evans   | PASS   | 2012-01-03
 1       | Mary | Lee     | FAIL   | 2012-01-04
 ...     | ...  | ...     | ...    | ...
 2       | John | Doe     | FAIL   | 2012-02-01 <--
 2       | Ryan | Smith   | FAIL   | 2012-02-02
 2       | Ann  | Evans   | FAIL   | 2012-02-03
 2       | Mary | Lee     | PASS   | 2012-02-04
 ...     | ...  | ...     | ...    | ...
 3       | John | Doe     | FAIL   | 2012-03-01
 3       | Ryan | Smith   | FAIL   | 2012-03-02
 3       | Ann  | Evans   | PASS   | 2012-03-03
 3       | Mary | Lee     | FAIL   | 2012-03-04 <--

请注意，exam_no和date并不一定与我选择的那种示例有关。

现在，我需要做的查询如下：

从最新的考试（exam_no = 3）中找到所有失败的学生（John Doe，Ryan Smith和Mary Lee）。
对于这些学生中的每一个，找到一批连续失败的考试中的第一个的日期。另一种说法是：对于这些学生中的每一个，找到他们上次通过考试后第一次失败考试的日期。（看看表中的箭头）。

结果表应该是这样的：

 name | surname | date_since_failing
------+---------+--------------------
 John | Doe     | 2012-02-01
 Ryan | Smith   | 2012-01-02
 Mary | Lee     | 2012-03-04

如何执行此类查询？

感谢您的时间。

Answer 1

你可以利用这样一个事实，即如果有人通过了最近的考试，那么他们最近的考试就没有通过任何考试失败：因此问题因为最近一次考试而导致第一次考试失败：

SELECT   name, surname, MIN(date) date_since_fail
FROM     results NATURAL LEFT JOIN (
  SELECT   name, surname, MAX(date) lastpass
  FROM     results
  WHERE    result = 'PASS'
  GROUP BY name, surname
) t
WHERE    result = 'FAIL' AND date > IFNULL(lastpass,0)
GROUP BY name, surname

在sqlfiddle上查看。

Answer 2

我应该使用获取最后一次通过考试的子查询，像是这样的事情：

SET @query_exam_no = 3;
SELECT
 name,
 surname,
 MIN(IF(date > last_passed_exam, date, NULL)) AS date_failing_since
FROM
 exam_results
 LEFT JOIN (
  SELECT
   name,
   surname, 
   MAX(date) AS last_passed_exam 
  FROM exam_results 
  WHERE result = 'PASS' 
  GROUP BY name, surname
 ) AS last_passed_exams USING (name, surname)
HAVING
 MAX(IF(exam_no = @query_exam_no, result, NULL)) = 'FAIL'
GROUP BY name, surname

Answer 3

这就够了：

select t.name,
       t.surname,
       t.date as 'date_since_failing'
from tablename t
inner join
(
    select name,
           surname,
           max(exam_no) as exam_no
    from tablename
    group by name, surname
    having min(result) = 'FAIL'
) aux on t.name = aux.name and t.surname = aux.surname and t.exam_no = aux.exam_no

Answer 4

如果没有它，你要求的条件是没有任何好处。这是工作示例。

select
  e.name,
  e.sur_name,
  min(e.date) as `LastFailed`
from exams as e
where e.result = 'Fail'
group by e.name
    order by e.name

这会产生此结果

name            sur_name    LastFailed
Ann         Evans       2012-02-03
John        Doe         2012-02-01
Mary        Lee         2012-01-04
Ryan        Smith       2012-01-02

涉及连续值的棘手SQL查询

4 个答案: