在 MySQL 中使用 EXISTS 进行查询的两个条件答案

【问题标题】：Two conditions for query in MySQL with EXISTS在 MySQL 中使用 EXISTS 进行查询的两个条件
【发布时间】：2016-10-02 19:56:21
【问题描述】：

我正在尝试获得满足 EXISTS 条件的两个条件的作者。但是，当子查询实际工作时，我会不断获取作者表中的所有条目。谁能告诉我我的查询到底有什么问题？我目前正在统计条目的数量。

SELECT COUNT(*) 
FROM author
WHERE 
EXISTS (SELECT A.author_id, A.author_name
FROM author AS A 
INNER JOIN author_publication AS AP ON A.author_id = AP.author_id
INNER JOIN publication AS P ON AP.pub_id = P.pub_id
WHERE P.pub_key LIKE '%/pvldb/%'
GROUP BY A.author_id
HAVING COUNT(*)>=10)
AND
EXISTS (SELECT A.author_id, A.author_name
FROM author AS A 
INNER JOIN author_publication AS AP ON A.author_id = AP.author_id
INNER JOIN publication AS P ON AP.pub_id = P.pub_id
WHERE P.pub_key LIKE '%/sigmod/%'
GROUP BY A.author_id
HAVING COUNT(*)>=10)
;

【问题讨论】：

标签： mysql exists

【解决方案1】：

您的子查询与您的主查询没有任何关系。因此，对于每条记录，您都会问同样的问题：“表中是否存在至少有十篇特定类型出版物的作者？”。每条记录的答案都是相同的，因此您可以取回所有记录。您真正想问的是：“表中是否存在至少十篇特定类型的出版物此作者？”

SELECT COUNT(*) 
FROM author
WHERE EXISTS 
(
  SELECT 1
  FROM author_publication AS AP 
  INNER JOIN publication AS P ON AP.pub_id = P.pub_id
  WHERE AP.author_id = author.author_id
  AND P.pub_key LIKE '%/pvldb/%'
  GROUP BY AP.author_id
  HAVING COUNT(*) >= 10
) 
AND EXISTS 
(
  SELECT 1
  FROM author_publication AS AP 
  INNER JOIN publication AS P ON AP.pub_id = P.pub_id
  WHERE AP.author_id = author.author_id
  AND P.pub_key LIKE '%/sigmod/%'
  GROUP BY AP.author_id
  HAVING COUNT(*) >= 10
);

如果要编写不相关的子查询，请改用IN：

SELECT COUNT(*) 
FROM author
WHERE author_id IN 
(
  SELECT AP.author_id
  FROM author_publication AS AP 
  INNER JOIN publication AS P ON AP.pub_id = P.pub_id
  WHERE P.pub_key LIKE '%/pvldb/%'
  GROUP BY AP.author_id
  HAVING COUNT(*) >= 10
) 
AND author_id IN 
(
  SELECT AP.author_id
  FROM author_publication AS AP 
  INNER JOIN publication AS P ON AP.pub_id = P.pub_id
  WHERE P.pub_key LIKE '%/sigmod/%'
  GROUP BY A.author_id
  HAVING COUNT(*) >= 10
);

下面是如何使用单个子查询来做到这一点：

SELECT COUNT(*) 
FROM
(
  SELECT AP.author_id
  FROM author_publication AS AP 
  INNER JOIN publication AS P ON AP.pub_id = P.pub_id
  WHERE P.pub_key LIKE '%/pvldb/%' OR P.pub_key LIKE '%/sigmod/%'
  GROUP BY AP.author_id
  HAVING COUNT(CASE WHEN P.pub_key LIKE '%/pvldb/%' THEN 1 END) >= 10
     AND COUNT(CASE WHEN P.pub_key LIKE '%/sigmod/%' THEN 1 END) >= 10
) found_authors;

【讨论】：

感谢您的解释！这对我来说真的很清楚。

【解决方案2】：

试试：

SELECT COUNT(A.author_id)
FROM
  author AS A
    INNER JOIN author_publication AS AP ON A.author_id = AP.author_id
    INNER JOIN publication AS P ON AP.pub_id = P.pub_id
WHERE
  P.pub_key LIKE '%/pvldb/%' OR P.pub_key LIKE '%/sigmod/%'
GROUP BY A.author_id
HAVING COUNT(A.author_id) >= 10;

【讨论】：

【解决方案3】：

您只需要将子查询关联到主查询：

SELECT COUNT(*) 
FROM author as AA
WHERE 
EXISTS (SELECT A.author_id, A.author_name
FROM author AS A 
INNER JOIN author_publication AS AP ON A.author_id = AP.author_id
INNER JOIN publication AS P ON AP.pub_id = P.pub_id
WHERE P.pub_key LIKE '%/pvldb/%'
AND AA.author_id=A.author_id
GROUP BY A.author_id
HAVING COUNT(*)>=10)
AND
EXISTS (SELECT A.author_id, A.author_name
FROM author AS A 
INNER JOIN author_publication AS AP ON A.author_id = AP.author_id
INNER JOIN publication AS P ON AP.pub_id = P.pub_id
WHERE P.pub_key LIKE '%/sigmod/%'
AND AA.author_id=A.author_id
GROUP BY A.author_id
HAVING COUNT(*)>=10)
;

【讨论】：