Question

此查询有什么问题？

SELECT *, (SELECT COUNT(*)
FROM
(
    SELECT NULL
    FROM words
    WHERE project=projects.id
    GROUP BY word
    HAVING COUNT(*) > 1
) T1) FROM projects

MySQL在'where子句'

中返回1054 Unknown列'projects.id'

由于

Answer 1

这有用吗？

SELECT *, (SELECT COUNT(*)
    FROM words
    WHERE words.project=projects.id) as pCount
 FROM projects

Answer 2

您的内部子查询对外部查询一无所知，因此projects表不可用。

Answer 3

看起来您正在尝试为每个项目计算多次出现的单词数。

您可以为所有项目运行子查询，然后使用JOIN从项目表中获取其余数据：

SELECT projects.*, COUNT(word) AS cnt
FROM projects
LEFT JOIN (
        SELECT project, word
        FROM words
        GROUP BY project, word
        HAVING COUNT(*) > 1
) T1
ON T1.project = projects.id
GROUP BY projects.id

结果：

测试数据：

CREATE TABLE projects (id INT NOT NULL);
INSERT INTO projects (id) VALUES (1), (2), (3);

CREATE TABLE words (project INT NOT NULL, word VARCHAR(100) NOT NULL);
INSERT INTO words (project, word) VALUES
(1, 'a'),
(2, 'a'),
(2, 'b'),
(2, 'b'),
(3, 'b'),
(3, 'b'),
(3, 'c'),
(3, 'c');

计算每一行

3 个答案: