Avg Group By 按班级统计平均分

AVG sql command is used to find out average value of a numeric field in a table.  Now let us apply this AVG command to this table and find out the average  mark obtain by all the students. The AVG command will calculate the average value  of all the marks

id	name	class	mark
1	John Deo	Four	75
2	Max Ruin	Three	85
3	Arnold	Three	55
4	Krish Star	Four	60
5	John Mike	Four	60
6	Alex John	Four	55

  Same way we can get the minimum value of a range of records by using SQL MIN command. Also check up SQL MAX command to get highest value of data
We will apply the AVG command here like this to the field  mark

SELECT avg( mark ) FROM `student`

avg(mark)

65.0000

The command will calculate average value of marks considering all the marks of the table. We can define some header  like this also.
SELECT AVG(mark) as avg_mark FROM `student` 

avg_mark

65.0000

Here we can use the Group By command to find out the average  mark obtained by each classes. 

SELECT class, avg( mark ) as avg_mark FROM `student` GROUP BY class

class	avg_mark
Four	62.5000
Three	70.0000

Please see the SQL SUM command to know the details on uses of GROUP BY command and the precautions. 

We can add condition to the sql command to get our desired result. We can add one Where clause to the query to consider records for which mark is more than some value ( say 55 )

SELECT avg( mark ) as avg_mark, class FROM student where mark > 55 GROUP BY class

avg_mark	class
65.0000	Four
85.0000	Three

Let us learn about the SUM sql command and how to use it in our tables. SUM command can be applied to numeric field and the total of the value is returned. Now let us apply this SUM command to this table and find out the total mark obtain by all the students. The SUM command will add all the values of the mark field and return to us. This is our table

id	name	class	mark
1	John Deo	Four	75
2	Max Ruin	Three	85
3	Arnold	Three	55
4	Krish Star	Four	60
5	John Mike	Four	60
6	Alex John	Four	55

 
We will apply the SUM command here like this to the field mark

SELECT sum( mark ) FROM `student`

sum(mark)

390

The command added all the values of the mark field and displayed. We can define some header  like this also.

SELECT SUM(mark) as total_mark FROM `student` 

total_mark

390

Now let us find out what is the total mark in each class. Here we can use the Group By command to find out the total mark obtained by each class

SELECT class, sum( mark ) as total_mark FROM `student` GROUP BY class

class	total_mark
Four	250
Three	140

You can see above that total mark of each class is displayed. Since we have two class in our table so the sql command has returned two class with total marks in each of them. We have to use Group By clause if we ask for the query to return any other field name other than the sum. Otherwise system will generate error.

We can add condition to the sql command to get our desired result. We can add one Where clause to the query to consider records for which mark is more than some value ( say 55 )

SELECT sum( mark ) as total_mark, class FROM student where mark > 55 GROUP BY class

total_mark	class
195	Four
85	Three

Other sql commands like between can be used along with this sum command to find out required results.