MySQL分层IF语句

Question

我正在尝试完成这段代码:

ORDER BY IF(j.groups IS NULL OR j.groups = '',  IF(j.title IS NULL, i.title), j.groups)

在英文中:如果存在则先按j.groups排序，如果存在则按j.title排序，最后按i.title排序。但上述方法行不通。

Answer 1

试试这个:

ORDER BY IF(j.groups IS NULL OR j.groups = '', IFNULL(j.title, i.title), j.groups);

或

ORDER BY COALESCE(j.groups, j.title, i.title);

或

ORDER BY IF(j.groups IS NULL OR j.groups = '', IF(j.title IS NULL, i.title, 1), j.groups);

Answer 2

您忘记了内部IF的else部分。试试这个:

ORDER BY IF(
  j.groups IS NULL OR j.groups = '',
  IF(
    j.title IS NULL,
    i.title,
    j.title -- this was missing
  ),
  j.groups
)

但是我发现这个语法更容易读:

ORDER BY COALESCE(IF(j.groups = '', NULL, j.groups), j.title, i.title)