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  • mysql行转列(列不固定)

    使用场景:由于题目的选项按行存储在数据库中,并且选项数量不是固定的
    image

    使用sql语句如下

    SELECT
      questionitem.questionId,
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 1, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 1), ',', -1), '') AS '1',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 2, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 2), ',', -1), '') AS '2',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 3, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 3), ',', -1), '') AS '3',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 4, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 4), ',', -1), '') AS '4',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 5, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 5), ',', -1), '') AS '5',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 6, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 6), ',', -1), '') AS '6',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 7, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 7), ',', -1), '') AS '7',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 8, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 8), ',', -1), '') AS '8',
      IF(length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 >= 9, 
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 9), ',', -1), '') AS '9'
    FROM
      questionitem
    GROUP BY
      questionitem.questionId;
    

    查询结果如下
    image

    sql分析:

    SELECT
      questionitem.questionId,
      GROUP_CONCAT(questionitem.content) as OPTION_LIST,
      length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 AS OPTION_LEN,
      SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 2) as '获取到第二个逗号的记录',
      SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 2), ',', -1) as '获取到第二个逗号的记录的最后一个'
    FROM
      questionitem
    GROUP BY
      questionitem.questionId;
    

    image

    函数
    SUBSTRING_INDEX

    -- 求GROUP_CONCAT的元素个数 把,替换为空然后做减法
    length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 AS OPTION_LEN
    
    -- 获取指定的第几个元素,先获取第三个逗号左侧全部字符串,在获取右侧第一个逗号右侧全部字符串
    SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 3), ',', -1) as '获取到第三个逗号的记录的最后一个'
    

    参考:
    https://blog.csdn.net/weixin_45716993/article/details/103234724

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  • 原文地址:https://www.cnblogs.com/aeolian/p/15378825.html
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