mysql行轉列(列不固定)

使用場景：由於題目的選項按行存儲在數據庫中，並且選項數量不是固定的

使用sql語句如下

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;

查詢結果如下

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 '獲取到第二個逗號的記錄',
  SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 2), ',', -1) as '獲取到第二個逗號的記錄的最后一個'
FROM
  questionitem
GROUP BY
  questionitem.questionId;

函數
SUBSTRING_INDEX

-- 求GROUP_CONCAT的元素個數 把,替換為空然后做減法
length(GROUP_CONCAT(questionitem.content))-length(replace(GROUP_CONCAT(questionitem.content),',','')) + 1 AS OPTION_LEN

-- 獲取指定的第幾個元素,先獲取第三個逗號左側全部字符串,在獲取右側第一個逗號右側全部字符串
SUBSTRING_INDEX(SUBSTRING_INDEX(GROUP_CONCAT(questionitem.content), ',', 3), ',', -1) as '獲取到第三個逗號的記錄的最后一個'

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