GROUP BY具有MAX日期

Question

执行此代码时遇到问题:

SELECT * FROM tblpm n 
WHERE date_updated=(SELECT MAX(date_updated) 
FROM tblpm GROUP BY control_number 
HAVING control_number=n.control_number)

基本上,我想返回每个控制号的最新日期.上面的查询返回正确的输出,但需要37秒.在输出显示之前.

是否还有其他sql子句或命令可以比上面的查询执行得更快？

提前致谢.

Answer 1

将子查询放在WHERE子句中并将其限制为n.control_number意味着它多次运行子查询.这称为相关子查询,它通常是性能杀手.

最好在FROM子句中运行子查询一次,以获得每个控件号的最大日期.

SELECT n.* 
FROM tblpm n 
INNER JOIN (
  SELECT control_number, MAX(date_updated) AS date_updated
  FROM tblpm GROUP BY control_number
) AS max USING (control_number, date_updated);

Answer 2

没有必要在子查询中分组...... where子句就足够了:

SELECT * FROM tblpm n
WHERE date_updated=(SELECT MAX(date_updated)
    FROM tblpm WHERE control_number=n.control_number)

另外,你有'date_updated'列的索引吗？那肯定会有所帮助.