Question

输入两个表格TABLE_A和TABLE_B

 TABLE_A       TABLE_B
 A_ID |   A    A_ID |  B        
    1 |   a       1 |  b
    2 |  a1       1 | b1
    3 |  a2       2 | b2

期望输出TABLE C

 TABLE_C
 A_ID |  A | C
    3 | a2 | NULL  <--- NULL if no matched A_ID in TABLE B
    1 |  a | b,b1  <--- Concat all rows in TABLE B with ','
    2 | a1 | b2

以下代码几乎可以提供上述TABLE_C，但我希望首先使用C对NULL字段DESC进行排序，然后ORDER BY C IS NULL DESC。 C似乎不起作用。请注意，如果NULL为C，则无论字段SELECT A1.A_ID, A1.A, GROUP_CONCAT(B1.B SEPARATOR ',') as 'C' FROM `TABLE_A` A1 LEFT JOIN `TABLE_B` B1 ON A1.A_ID=B1.A_ID GROUP BY A1.A_ID, A1.A;中的值如何，TABLE_C将按A_ID排序。

SELECT 
  A1.A_ID, 
  A1.A, 
  GROUP_CONCAT(B1.B SEPARATOR ',') as 'C'
FROM `TABLE_A` A1 
  LEFT JOIN `TABLE_B` B1 
  ON A1.A_ID=B1.A_ID 
GROUP BY A1.A_ID, A1.A
ORDER BY C IS NULL DESC, A1.A_ID; <--- Order by C with NULL failed.

以下SQL给出了错误。

d = feedparser.parse('http://feeds.bbci.co.uk/news/rss.xml')
for entry in d.entries:
    print(entry.media_thumbnail)

参考＆＃39; C＆＃39;不支持（参考群组功能）

Answer 1

先按ORDER BY Null值，然后按A1.A_ID使用：

SELECT A1.A_ID, A1.A, GROUP_CONCAT(B1.B SEPARATOR ',') as C 
FROM `TABLE_A` A1 
LEFT JOIN `TABLE_B` B1 ON A1.A_ID=B1.A_ID 
GROUP BY A1.A_ID, A1.A 
ORDER BY (CASE WHEN GROUP_CONCAT(B1.B SEPARATOR ',') IS NULL then GROUP_CONCAT(B1.B SEPARATOR ',') ELSE A1.A_ID END) ;

关于错误Reference 'C' not supported (reference to group function)，您应该按'C'排序，即

ORDER BY 'C' IS NULL DESC, A1.A_ID;

而不是ORDER BY C IS NULL DESC, A1.A_ID;

Answer 2

您无法按顺序使用Is Null

SELECT 
  A1.A_ID, 
  A1.A, 
  GROUP_CONCAT(B1.B SEPARATOR ',') as 'C'
FROM `TABLE_A` A1 
  LEFT JOIN `TABLE_B` B1 
  ON A1.A_ID=B1.A_ID 
GROUP BY A1.A_ID, A1.A
ORDER BY C, A1.A_ID DESC

由GROUP_CONCAT（）创建的MySQL ORDER BY字段

2 个答案: