mysql - 按类别获取具有第二个最近时间戳的行？

Question

我有一个 MySQL 表（称为formstatus），它有 4 列：id、form、status和timestamp，id作为主键，form和status一起唯一。我想获得status具有第二个最近时间戳的行集（特别是）form。我试过

SELECT form, status FROM (
  SELECT form, status, timestamp
  FROM formstatus
  WHERE timestamp < max(timestamp)
  GROUP BY form) as a
WHERE timestamp = max(timestamp)
GROUP BY form

但它不起作用，因为显然在 where 子句中不允许聚合。那么，是否有一个查询可以返回我正在寻找的信息？

Answer 1

试试这个查询 -

SELECT
  t1.*, COUNT(*) pos
FROM formstatus t1
  LEFT JOIN formstatus t2
    ON t2.form = t1.form AND t2.timestamp >= t1.timestamp
GROUP BY
  t1.form, t1.timestamp
HAVING
  pos = 2;

Answer 2

生成错误是因为您不能在WHERE子句中使用聚合。尝试将您的条件移至HAVING子句。

GROUP BY form
HAVING timestamp < max(timestamp)