Finding the highest average salary of a department using joins in SQL? - sql

Question

I have two tables with the below schema:
Table 1
-------
empID
empName
Table 2
-------
empID
department
salary
Assuming the tables are:
Table1:
empID|empName
1 A
2 B
3 C
4 D
5 E
6 F
7 G
8 H
9 I
10 J
Table 2:
empID|department|salary
1 X 10
2 X 10
3 X 10
4 Y 5
5 Y 5
6 Y 5
7 Y 5
8 Y 5
9 Z 3
10 Z 3
I need to find the department name with the highest average salary and display them along with the employee names.
The output I am expecting is:
empName|department|salary
A 10
B X 10
C 10
This was an interview question, and I am recreating this from memory so it might not be perfect. I am also picking up SQL after a gap of more than 2 years. Please suggest if I am missing something.
The query that I have formed is:
SELECT
table1.empName,
TOP(1) AVG(table2.salary),
table2.department
FROM
table1
INNER JOIN
table2
ON table1.empID = table2.empID
GROUP BY
table2.department

Answer 1

Your syntax looks like SQL Server (the "TOP 1"). In that database, I would do something like this:
SELECT TOP (1) WITH TIES t1.empName, t2.salary, t2.department
FROM table1 t1 INNER JOIN
table2 t2
ON t1.empID = t2.empID
ORDER BY AVG(t2.salary) OVER (PARTITION BY t2.department) DESC;
A more generic solution:
SELECT empName, salary, department
FROM (SELECT t.*,
DENSE_RANK() OVER (ORDER BY avg_salary) as seqnum
FROM (SELECT t1.empName, t2.salary, t2.department,
AVG(t2.salary) OVER (PARTITION BY t2.department) as avg_salary
FROM table1 t1 INNER JOIN
table2 t2
ON t1.empID = t2.empID
) t
) t
WHERE seqnum = 1;

Answer 2

Based on what I draw from your question, I would approach it this way.
WITH department_rank AS
(
SELECT
department,
RANK() OVER(ORDER BY avg_salary DESC) AS avg_salary_rank
FROM
(
SELECT
department,
AVG(salary) AS avg_salary
FROM
table2
GROUP BY
department
) tbl
)
SELECT
dept.department,
emp.empID,
emp.empName,
dept.salary
FROM
table2 dept
JOIN
table1 emp
ON (emp.empID = dept.empID)
JOIN
department_rank drnk
ON (drnk.department = dept.department)
AND (drnk.avg_salary_rank = 1) --Top ranked department based on average salary
OUTPUT: