在 MySQL 中使用 EXISTS 进行查询的两个条件
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【中文标题】在 MySQL 中使用 EXISTS 进行查询的两个条件【英文标题】:Two conditions for query in MySQL with EXISTS 【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】:
【参考方案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。【参考方案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】:
【参考方案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】:
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