mysql - ORDER BY 无法按预期使用 GROUP BY

Question

我有两个表，establishment 和 branch。建立是父表。当我选择一个分支列表时，如果分支标题为空，我希望它检索机构名称。另外，我只想为每个机构显示一次分支机构。这是我想出的:

SELECT 
  branch.id
  , branch.establishment_ID
  , IFNULL(branch.branch_title, establishment.name) AS branch_title 
FROM branch 
LEFT JOIN establishment ON branch.establishment_ID = establishment.id 
WHERE cityID = 2 
GROUP BY establishment_ID 
ORDER BY branch_title

但是，结果似乎没有任何特定顺序。我希望它们按字母顺序排列。我读过 MySQL 5.0.5 在同一个查询中有 GROUP BY 和 ORDER BY 的问题，但我正在使用 5.5.9。我该怎么做才能修复此查询？

Answer 1

我猜(了解其他 SQL 系统)您的 ORDER BY 正在应用于 branch.branch_title，而不是您的新别名。您能否将 IFNULL() 表达式复制到 ORDER BY 中？

SELECT 
  branch.id
  , branch.establishment_ID
  , IFNULL(branch.branch_title, establishment.name) AS branch_title 
FROM branch 
LEFT JOIN establishment ON branch.establishment_ID = establishment.id 
WHERE cityID = 2 
GROUP BY establishment_ID 
ORDER BY IFNULL(branch.branch_title, establishment.name)

否则，NULL branch_title 行将始终出现(之前/之后)非NULL 行，并且不会以任何方式排序。