I am new to SQL.
I checked "another solutionSQL JOIN two tables with AVG" posted in StackOverflow. And I don't get the meaning with this line in that article:AVG(score.score * 1.0) Besides, the alternative solution below doesn't work at all:
SELECT songs.id, songs.song, songs.artist,
(SELECT AVG(Score) FROM score WHERE score.id = songs.id) AS AvgScore)
FROM songs
Here are my tables:
[employees]
Dep ID | SALARY
1 | 500
2 | 200
1 | 300
2 | 1000
2 | 400
3 | 200
3 | 300
[departments]
Dept ID Dep| Dept Name
1 | Volcano
2 | ShootingStar
3 | Tsunami
In the end, I want to create a list looks like:
Dept Name | Average Salary
Volcano | $$$
ShootingStar| $$
Tsunami | $$$$
I tried various ways and hunting hints in stackoverflow for sub queries/inner join features but still can't get it.
Based on the solution in the previous link SQL JOIN two tables with AVG, this code works:
-- mapping DEPT ID with NAME + average salary by DEPT --
select EMPLOYEES.DEP_ID, DEPARTMENTS.DEP_NAME, AVG(EMPLOYEES.SALARY) as AVG_S
from EMPLOYEES
LEFT JOIN DEPARTMENTS
ON EMPLOYEES.DEP_ID = DEPARTMENTS.DEPT_ID_DEP
group by DEP_ID, DEP_NAME;
However, I want to understand the reason WHY my original one doesn't work?
select E.DEP_ID, D.DEP_NAME, (select AVG(SALARY) from EMPLOYEES group by DEP_ID) as AVG_S
from EMPLOYEES E, DEPARTMENTS D
where E.DEP_ID = D.DEPT_ID_DEP
group by DEP_ID, DEP_NAME;
Please help!
Thank you very much.
The query you wanted to write:
select
e.dep_id,
d.dep_name,
(select avg(salary) from employees e where e.dep_id = d.dept_id_dep) as avg_s
from departments d;
The logic of the query is to select from departments only, then use a correlated subquery to compute the average salaries of employees of the department. This avois aggregating in the outer query.
Your query fails in the following regards:
table
employeesis in thefromclause of the outer querythe outer query does
group bythe subquery is not corelated on the department