MySQL: A small practice for where/group by/having/order by clause

# Here is an example for five sql caluse
# Pay attention to the sequence of the four clause
# select *** from *** where *** group by *** having *** order by *** limit ***
select stu_name, avg(stu_score) as avg_score from stu where stu_name != (select stu_name from stu where stu_score = 90) group by stu_name having avg_score> 30 order by avg_score desc;

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# Examples are as below
# 1. Create a table
create table stu (stu_name varchar(20), stu_course varchar(20), stu_score int);

# 2. Insert data
mysql> insert into stu values('zhangsan', 'Math', 90);
Query OK, 1 row affected

mysql> insert into stu values('zhangsan', 'Literature', 50);
Query OK, 1 row affected

mysql> insert into stu values('zhangsan', 'Geograph', 40);
Query OK, 1 row affected

mysql> insert into stu values('lisi', 'Literature', 55);
Query OK, 1 row affected

mysql> insert into stu values('lisi', 'Politic', 4
5);
Query OK, 1 row affected

mysql> insert into stu values('wangwu', 'Politic', 30);
Query OK, 1 row affected

# 3. Purpose: Try to find the average score for the students whose invalid course count is bigger than 2.

## 3.1 Try to find the average score for all the students
select stu_name, avg(stu_score) from stu group by stu_name;

## 3.2 Try to find the average score for the students whose invalid course count is bigger than 2
select stu_name, avg(stu_score) as avg_score, sum(stu_score < 60) as invalid_count from stu group by stu_name having invalid_count >= 2;