Select 行按具有最大聚合的列分组

Question

给定以下数据集，我如何找到最多 ApplicationID 有 "Accepted" 决定的参考电子邮件地址？

CREATE TABLE IF NOT EXISTS `EmailReferences` (
  `ApplicationID` INT NOT NULL,
  `Email` VARCHAR(45) NOT NULL,
  PRIMARY KEY (`ApplicationID`, `Email`)
);
INSERT INTO EmailReferences (ApplicationID, Email)
VALUES
(1, 'ref10@test.org'), (1, 'ref11@test.org'), (1, 'ref12@test.org'),
(2, 'ref20@test.org'), (2, 'ref21@test.org'), (2, 'ref22@test.org'),
(3, 'ref11@test.org'), (3, 'ref31@test.org'), (3, 'ref32@test.org'),
(4, 'ref40@test.org'), (4, 'ref41@test.org'), (4, 'ref42@test.org'),
(5, 'ref50@test.org'), (5, 'ref51@test.org'), (5, 'ref52@test.org'),
(6, 'ref60@test.org'), (6, 'ref11@test.org'), (6, 'ref62@test.org'),
(7, 'ref70@test.org'), (7, 'ref71@test.org'), (7, 'ref72@test.org'),
(8, 'ref10@test.org'), (8, 'ref81@test.org'), (8, 'ref82@test.org')
;

CREATE TABLE IF NOT EXISTS `FinalDecision` (
  `ApplicationID` INT NOT NULL,
  `Decision` ENUM('Accepted', 'Denied') NOT NULL,
  PRIMARY KEY (`ApplicationID`)
);
INSERT INTO FinalDecision (ApplicationID, Decision)
VALUES
(1, 'Accepted'), (2, 'Denied'),
(3, 'Accepted'), (4, 'Denied'),
(5, 'Denied'),   (6, 'Denied'),
(7, 'Denied'),   (8, 'Accepted')
;

Fiddle 相同：http://sqlfiddle.com/#!9/03bcf2/1

最初，我使用 LIMIT 1 和 ORDER BY CountDecision DESC，像这样：

SELECT  er.email, COUNT(fd.Decision) AS CountDecision
FROM    EmailReferences AS er
JOIN    FinalDecision AS fd ON er.ApplicationID = fd.ApplicationID
WHERE   fd.Decision = 'Accepted'
GROUP   BY er.email
ORDER   BY CountDecision DESC
LIMIT   1
;

然而，我突然想到我可以有多个电子邮件地址，这些地址涉及不同的 "most accepted" 决定（即平局，可以这么说），并且这些将被过滤掉（是正确的措辞吗？）与 LIMIT 关键字。

然后我尝试了上述查询的变体，将 ORDER BY 和 LIMIT 行替换为：

HAVING MAX(CountDecision)

但我意识到这只是陈述的一半：MAX(CountDecision) 需要与某物进行比较。我只是不知道是什么。

任何指点将不胜感激。谢谢！

注意：这是作业。

更新： 明确地说，我正在尝试从 EmailReferences 中查找 Email 的值和计数。但是，我只想要具有 FinalDecision.Decision = 'Accepted' 的行（在匹配 ApplicantID 时）。根据我的数据，结果应该是：

Email          | CountDecision
---------------+--------------
ref10@test.org | 2
ref11@test.org | 2

Answer 1

MySQL 仍然缺少 window 功能，但是当版本 8 准备好生产时，这会变得更容易。所以为了将来的参考，或者对于像 Mariadb 这样已经有 window 函数的数据库：

CREATE TABLE IF NOT EXISTS `EmailReferences` (
  `ApplicationID` INT NOT NULL,
  `Email` VARCHAR(45) NOT NULL,
  PRIMARY KEY (`ApplicationID`, `Email`)
);

INSERT INTO EmailReferences (ApplicationID, Email)
VALUES
(1, 'ref10@test.org'), (1, 'ref11@test.org'), (1, 'ref12@test.org'),
(2, 'ref20@test.org'), (2, 'ref21@test.org'), (2, 'ref22@test.org'),
(3, 'ref30@test.org'), (3, 'ref31@test.org'), (3, 'ref32@test.org'),
(4, 'ref40@test.org'), (4, 'ref41@test.org'), (4, 'ref42@test.org'),
(5, 'ref50@test.org'), (5, 'ref51@test.org'), (5, 'ref52@test.org'),
(6, 'ref60@test.org'), (6, 'ref11@test.org'), (6, 'ref62@test.org'),
(7, 'ref70@test.org'), (7, 'ref71@test.org'), (7, 'ref72@test.org'),
(8, 'ref10@test.org'), (8, 'ref81@test.org'), (8, 'ref82@test.org')
;

CREATE TABLE IF NOT EXISTS `FinalDecision` (
  `ApplicationID` INT NOT NULL,
  `Decision` ENUM('Accepted', 'Denied') NOT NULL,
  PRIMARY KEY (`ApplicationID`)
);

INSERT INTO FinalDecision (ApplicationID, Decision)
VALUES
(1, 'Accepted'), (2, 'Denied'),
(3, 'Accepted'), (4, 'Denied'),
(5, 'Denied'),   (6, 'Denied'),
(7, 'Denied'),   (8, 'Accepted')
;

select email, CountDecision
from (
     SELECT   er.email, COUNT(fd.Decision) AS CountDecision
            , max(COUNT(fd.Decision)) over() maxCountDecision
     FROM EmailReferences AS er
     JOIN FinalDecision AS fd ON er.ApplicationID = fd.ApplicationID
     WHERE    fd.Decision = 'Accepted'
     GROUP    BY er.email
     ) d
where CountDecision = maxCountDecision

email          | CountDecision
:------------- | ------------:
ref10@test.org |             2

dbfiddle here

Answer 2

例如...

SELECT a.*
  FROM 
     ( SELECT x.email
            , COUNT(*) total
         FROM emailreferences x
         JOIN finaldecision y
           ON y.applicationid = x.applicationid
        WHERE y.decision = 'accepted'
        GROUP
           BY x.email
     ) a
  JOIN
     ( SELECT COUNT(*) total
         FROM emailreferences x
         JOIN finaldecision y
           ON y.applicationid = x.applicationid
        WHERE y.decision = 'accepted'
        GROUP
           BY x.email
        ORDER 
           BY total DESC 
        LIMIT 1
     ) b
    ON b.total = a.total;

Select 行按具有最大聚合的列分组

Select rows grouped by a column having max aggregate

mysql

group-by

having