MySQL - 操作数应包含1列

Question

在我正在创建的系统上工作时,我试图在我的项目中使用以下查询:

SELECT
topics.id,
topics.name,
topics.post_count,
topics.view_count,
COUNT( posts.solved_post ) AS solved_post,
(SELECT users.username AS posted_by,
    users.id AS posted_by_id
    FROM users
    WHERE users.id = posts.posted_by)
FROM topics
LEFT OUTER JOIN posts ON posts.topic_id = topics.id
WHERE topics.cat_id = :cat
GROUP BY topics.id

":cat"由我的PHP代码绑定,因为我正在使用PDO.2是":cat"的有效值.

该查询虽然给我一个错误:"#1241 - 操作数应包含1列"

让我感到困惑的是,我认为这个查询没有问题.选择列,然后从另一个表中再选择两个,然后从那里继续.我只是无法弄清楚问题是什么.

是否有一个简单的解决方法,或其他方式来编写我的查询？

Answer 1

您的子查询正在选择两列,而您正在使用它来投影一列(作为外部SELECT子句的一部分).在此上下文中,您只能从此类查询中选择一列.

考虑加入users表格; 在选择所需的列时,这将为您提供更大的灵活性users.

SELECT
topics.id,
topics.name,
topics.post_count,
topics.view_count,
COUNT( posts.solved_post ) AS solved_post,
users.username AS posted_by,
users.id AS posted_by_id

FROM topics

LEFT OUTER JOIN posts ON posts.topic_id = topics.id
LEFT OUTER JOIN users ON users.id = posts.posted_by

WHERE topics.cat_id = :cat
GROUP BY topics.id

Answer 2

如果您不小心使用逗号而不是AND在以下ON子句中使用逗号,也会发生此错误JOIN:

JOIN joined_table ON (joined_table.column = table.column, joined_table.column2 = table.column2)
                                                        ^
                                             should be AND, not a comma

Answer 3

COUNT( posts.solved_post ) AS solved_post,
(SELECT users.username AS posted_by,
    users.id AS posted_by_id
    FROM users
    WHERE users.id = posts.posted_by)

好吧,你不能从一个子查询中获得多个列.幸运的是,第二栏已经是posts.posted_by!所以:

SELECT
topics.id,
topics.name,
topics.post_count,
topics.view_count,
posts.posted_by
COUNT( posts.solved_post ) AS solved_post,
(SELECT users.username AS posted_by_username
    FROM users
    WHERE users.id = posts.posted_by)
...

Answer 4

如果不小心在子句中使用=而不是，也会发生此错误：INWHERE

例如：

WHERE product_id = (1,2,3);

Answer 5

就我而言，问题是我错误地用括号包围了我的列选择：

SELECT (p.column1, p.colum2, p.column3) FROM table1 p where p.column1 = 1;

并且必须是：

SELECT p.column1, p.colum2, p.column3 FROM table1 p where p.column1 = 1;

听起来很傻，但它导致了这个错误，需要一些时间才能弄清楚。