mysql - 如何在mysql表中选择，平均和排序

Question

我在 mySql 中有一个表，就像这张图片一样

我想编写一个查询，其结果将按 LESSON 列分组，并添加新行，即 LESSON 列的平均值和 CNT 列值的总和....
对于这个查询，我使用这个

我使用这个查询，但它给出的结果如图 3 所示，在这种情况下我无法按 PERC 排序

select no, STUD_ID,CLASS,LESSON, AVG(PERC) as PERC,SUM(CNT) as CNT from t_lesson where LESSON='CHEM' group by CLASS
union all
select no,STUD_ID,CLASS,'AVERAGE' as LESSON, AVG(PERC) as PERC, SUM(CNT) as CNT from t_lesson where LESSON='CHEM' group by LESSON

Answer 1

看起来您正在选择lessonis的所有行CHEM，然后您想要一个具有平均百分比的额外行。怎么样：

select *
from (
  -- this part gets all the "CHEM" rows
  select * 
  from <your_table_name>
  where lesson = "CHEM"
  union
  -- this parts selects the aggregate row
  select 
    NULL            as `no`, 
    NULL            as `stud_id`, 
    NULL            as `class`,
    "average"       as `lesson`,
    avg(percentage) as `perc`, 
    sum(count)      as `cnt`
  from <your_table_name>
  where lesson = "CHEM"
) q
order by `perc` desc;

请注意，排序是由外部查询执行的。

Answer 2

不知道你是否正在寻找这样的东西：

SELECT fields_list, (field_to_modify + (SELECT AVG(average_field) FROM table2)) AS order_field 
  FROM table1
WHERE your_conditions
  ORDER BY order_field;