sql query student results greater than the average

scene

There is a table, which contains the grades of students and their corresponding courses. It is necessary to find courses that are greater than the average of all courses of the student.

Table structure

Solution

I think of two kinds of SQL, as follows

The first

Find out the average value corresponding to each student and connect it with the original table, and then compare the course score and average value with the query conditions

select t1.student_id,course_id
from t_mark t1,
	(
	SELECT student_id,AVG(mark) avg 
	from 
	t_mark
	group by student_id
	) t2
where t1.student_id = t2.student_id
and t1.mark > t2.avg

The second

Each time you use a subquery in the query condition to find out the student’s course average comparison

select t1.student_id,course_id
from t_mark t1
where t1.mark>
	(select AVG(mark)
	 from t_mark t2
	 where t2. student_id = t1.student_id)

Compare two SQL

Data input

In order to have enough data for comparison, write a stored procedure to insert 3 pieces of score data for 3000 students

CREATE DEFINER=`root`@`localhost` PROCEDURE `insert_to_mark`()
BEGIN
	#Routine body goes here...
	DECLARE i INTEGER;
	SET i = 1;
	WHILE i<=3000 DO
		INSERT INTO t_mark 
		(student_id,course_id,mark)
		VALUES
		(i,1,RAND()*70+30),
		(i,2,RAND()*70+30),
		(i,3,RAND()*70+30);
		set i = i+1;
	END WHILE;
END

Inserted successfully

Query comparison

The first

The second

The gap is quite large.

in conclusion

It is better to use the first one. I don’t know if there is a more efficient query sql. Welcome to comment~~