MySQL 错误子查询返回多于 1 行

Question

I have the following query:

SELECT 
    users.*,
    classes.*, 
    evaluation.student_id, 
    evaluation.class_id, 
    evaluation.chapter_title,
    (SELECT
        `score`
     FROM 
        `evaluation`
     WHERE 
        `class_id` = 1 
     AND 
        `id` 
     IN
        (SELECT 
            MAX(`id`)
         FROM
            `evaluation`
         WHERE
            `class_id` = 1
         GROUP BY
            `chapter_title`)
     GROUP BY
        `chapter_title`) 
     AS
        `score`,
     (SELECT
        `total_score`
      FROM
        `evaluation`
      WHERE
        `class_id` = 1
      AND
        `id`
      IN
        (SELECT 
            MAX(`id`)
         FROM
            `evaluation`
         WHERE
            `class_id` = 1
         GROUP BY
            `chapter_title`)
      GROUP BY
         `chapter_title`)
      AS
         `total_score`
      FROM
        (`evaluation`
      INNER JOIN
        `users`
      ON 
        evaluation.student_id=users.id)
      INNER JOIN
        `classes`
      ON
        evaluation.class_id=classes.id
      WHERE
        users.role='student'
      AND
        evaluation.class_id = 1
      AND
        evaluation.student_id = 8

But when I execute this query in phpmyadmin it displays a error message saying:

#1242 - Subquery returns more than 1 row

Whats wrong in the query.Please help. Thanks in advance.

I have this tables:

users

classes

evaluation

In the evaluation table(last image).. I would only want to return a distinct chapter_title or a grouped chapter_title that has the highest id and has a student_id of 8.

I needed to use this query... but returns an error.

Answer 1

The problem is that both of your queries return more than one record:

SELECT `score`
     FROM `evaluation`
     WHERE `class_id` = 1 
       AND `id` IN (SELECT  MAX(`id`)
                     FROM `evaluation`
                     WHERE `class_id` = 1
                     GROUP BY `chapter_title`)
     GROUP BY `chapter_title`;

SELECT `total_score`
      FROM `evaluation`
      WHERE `class_id` = 1
        AND `id` IN (SELECT  MAX(`id`)
                     FROM `evaluation`
                     WHERE `class_id` = 1
                     GROUP BY `chapter_title`)
      GROUP BY`chapter_title`;

See SQL Fiddle with demo

I altered your query slightly to the following:

SELECT u.*,
    c.*, 
    e.student_id, 
    e.class_id, 
    e.chapter_title,
    (SELECT `score`
     FROM `evaluation` e1
     WHERE `class_id` = 1 
       AND e.id = e1.id
       AND `id` IN (SELECT  MAX(`id`)
                     FROM `evaluation`
                     WHERE `class_id` = 1
                     GROUP BY `chapter_title`)
     GROUP BY `chapter_title`)  AS`score`,
     (SELECT `total_score`
      FROM `evaluation` e1
      WHERE `class_id` = 1
        AND e.id = e1.id
        AND `id` IN (SELECT  MAX(`id`)
                     FROM `evaluation`
                     WHERE `class_id` = 1
                     GROUP BY `chapter_title`)
      GROUP BY`chapter_title`) AS `total_score`
FROM `evaluation` e
INNER JOIN `users` u
  ON e.student_id=u.id
INNER JOIN `classes` c
  ON e.class_id=c.id
WHERE u.role='student'
  AND e.class_id = 1
  AND e.student_id = 8

See SQL Fiddle with demo

Answer 2

You'll need to use LIMIT 1 on all of your subqueries, to ensure only one value is returned.

Eg

select users.*, classes.*, evaluation.student_id, evaluation.class_id, evaluation.chapter_title, (
        select `score`
        from `evaluation`
        where `class_id` = 1
            and `id` in (
                select MAX(`id`)
                from `evaluation`
                where `class_id` = 1
                group by `chapter_title`
                )
        group by `chapter_title`
        limit 1
        ) as `score`, (
        select `total_score`
        from `evaluation`
        where `class_id` = 1
            and `id` in (
                select MAX(`id`)
                from `evaluation`
                where `class_id` = 1
                group by `chapter_title`
                )
        group by `chapter_title`
        limit 1
        ) as `total_score`
from (
    `evaluation` inner join `users` on evaluation.student_id = users.id
    )
inner join `classes` on evaluation.class_id = classes.id
where users.role = 'student'
    and evaluation.class_id = 1
    and evaluation.student_id = 8

Answer 3

Here is the corrected fiddle :

SELECT
    users.*,
    classes.*,
    evaluation.student_id,
    evaluation.class_id,
    evaluation.chapter_title,
    evaluation.score,
    evaluation.total_score
 FROM
    evaluation
 INNER JOIN users
 ON
    evaluation.student_id=users.id
  INNER JOIN
    classes
  ON
    evaluation.class_id=classes.id
  WHERE
    users.role='student'
  AND
    evaluation.class_id = 1
  AND
    evaluation.student_id = 8
 AND
 EXISTS
    (SELECT
        *
     FROM
        evaluation
     WHERE
        class_id = 1
    HAVING MAX(id) = evaluation.id)
 GROUP BY
    chapter_title

Main changes:

1. Instead of IN , use EXISTS (should be faster since it won't need to return all of it's sub-rows);

2. Some subqueries weren't needed.

Answer 4

Most likely your SELECT scores subquery (first one) is returning more than one row. It's the only one where it's not used in an IN() clause, and its results are returned as a field in the parent query. Since it's "returning" to a field, it MUST result in a single value+single row result set.