how to select, average and sort in mysql table

问题

i have a table in mySql like in this picture

and i want to write a query which result will group by LESSON column, and add new row which is average value of LESSON column and sum CNT column values....
for this query i use this one

i use this query but it gives result like in picture 3 and i cant sort by PERC in this case

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

回答1:

It looks like you're selecting all of the rows where lesson is CHEM, and then you want an extra row with the average of the percantage. How about:

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;

Note that the sorting is performed by the outer query.

回答2:

don't know if you're looking for something like this:

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

来源：https://stackoverflow.com/questions/10123110/how-to-select-average-and-sort-in-mysql-table