Question

我试图在记录前30天进行查询以获取记录。我有两个表学生和学生参加跟踪。我试图在30天之前找到谁缺席。我的查询和结果。

SELECT * FROM student st, attend_track at 
WHERE can.can_status = 1 
    AND DATEDIFF(NOW(), DATE(st.created)) > 30
    AND st.std_id = st.std_id 
ORDER BY UNIX_TIMESTAMP(at.created) DESC

查询返回以下输出。

  ------------------------------------------------------
  | Student name        | Class              | Created |
  ------------------------------------------------------
  | AAA                 | XL                 | 21/02/18|
  ------------------------------------------------------
  | BBB                 | XL                 | 18/02/18|
  ------------------------------------------------------
  | CCC                 | XL                 | 12/02/18|
  ------------------------------------------------------
  | DDD                 | XL                 | 08/02/18|
  ------------------------------------------------------
  | EEE                 | XL                 | 02/02/18|
  ------------------------------------------------------
  | FFF                 | XL                 | 22/01/18|
  ------------------------------------------------------

但是那天下面的记录已经出现了。

  ------------------------------------------------------
  | Student name        | Class              | Created |
  ------------------------------------------------------
  | CCC                 | XL                 | 10/03/18|
  ------------------------------------------------------
  | DDD                 | XL                 | 20/03/18|
  ------------------------------------------------------
  | FFF                 | XL                 | 28/02/18|
  ------------------------------------------------------

我尝试了子查询NOT IN和<>，因为大数据（超过200k）不起作用。

SELECT * 
FROM student st, attend_track atr 
WHERE st.std_status = 1 
    AND DATEDIFF(NOW(), DATE(atr.created)) > 30 
    AND st.std_id = atr.std_id  
    AND st.std_id NOT IN (SELECT  atr.std_id
                      FROM attend_track atk 
                      WHERE DATEDIFF(NOW(), DATE(atk.created)) < 30) 
ORDER BY UNIX_TIMESTAMP(atr.created) DESC

请建议我如何优化查询。

Answer 1

试试这个：

SELECT *
FROM (SELECT std_id, MAX(atr.created) last_attended
        FROM attend_track atr
        GROUP BY std_id) lst
INNER JOIN student st ON st.std_id = lst.std_id
WHERE DATEDIFF(NOW(), DATE(last_attended)) > 30

这仅基于您的查询。我不知道表结构或预期输出。要提高此查询的性能，您应该查看索引。查看查询的EXPLAIN计划以查看问题区域。

如何在30天记录之前在mysql查询中添加where条件

1 个答案: