Question

我的查询

SELECT * FROM  semesters WHERE student_id = 434 AND marks_id <=  576 AND semester_id <=  2345

我有一个表“学期”我在MySQL中使用我的查询输出以下数据：

id       semester_id       exam_id      student_id   marks_id
1          2344               25            434       255
2          2344               34            434       575 
3          2345               48            434       576

如何使用我的查询将其变为以下格式？

 id          semester_id        exam_id      student_id   marks_id
 1             2344              25,34          434       255,575
 2             2345               48            434         576

我希望semster_id 2344有两个exam_ids，我想用逗号分隔两个exam_ids和marks_id在单行中。我必须使用GROUP_CONCAT，但我不确定它是如何工作的。提前谢谢。

Answer 1

将MySQL GROUP_CONCAT与group by

一起使用

SELECT *,GROUP_CONCAT(exam_id) as exam_id,GROUP_CONCAT(marks_id) 
as marks_id FROM  semesters group by semester_id

Answer 2

1*x + 0*y - 1 = 0

Answer 3

我会使用GROUP_CONCAT，但我会建议强制命令并处理列中的NULL值。因此，即使一个为空，考试和分数也应该匹配： -

SELECT semester_id,
    GROUP_CONCAT(IFNULL(exam_id, '') ORDER BY id),
    student_id,
    GROUP_CONCAT(IFNULL(marks_id, '') ORDER BY id)
FROM  semesters 
WHERE student_id = 434 
AND marks_id <=  576 
AND semester_id <=  2345
GROUP BY semester_id, 
        student_id

如果您需要序列ID，则可能需要使用变量手动创建它： -

SELECT  @cnt:=@cnt + 1 AS id,
        sub0.semester_id,
        sub0.exam_id_out,
        sub0.student_id,
        sub0.marks_id_out
FROM
(
    SELECT semester_id,
        GROUP_CONCAT(IFNULL(exam_id, '') ORDER BY id) AS exam_id_out,
        student_id,
        GROUP_CONCAT(IFNULL(marks_id, '') ORDER BY id) AS marks_id_out
    FROM  semesters 
    WHERE student_id = 434 
    AND marks_id <=  576 
    AND semester_id <=  2345
    GROUP BY semester_id, 
            student_id
    ORDER BY semester_id
) sub0
CROSS JOIN 
(
    SELECT @cnt:=0
) sub1

如何使用查询的两列GROUP_CONCAT

3 个答案: