How to use GROUP BY when fetching values from More than one Table [duplicate] - sql

Question

This question already has answers here:
Get top 1 row of each group
(19 answers)
Closed 4 months ago.
We have 2 Tables Employees and Department.
We want to show the maximum salary from each department and their corresponding employee name from the employee table and the department name from the department table.
Employee Table
EmpId | EmpName |salary |DeptId
101 shubh1 1000 1
101 shubh2 4000 1
102 shubh3 3000 2
102 shubh4 5000 2
103 shubh5 12000 3
103 shubh6 1000 3
104 shubh7 1400 4
104 shubh8 1000 4
Department Table
DeptId | DeptName
1 ComputerScience
2 Mechanical
3 Aeronautics
4 Civil
I tried doing it but was getting error
SELECT DeptName FROM Department where deptid IN(select MAX(salary),empname,deptid
FROM Employee
GROUP By Employee.deptid)
Error
Token error: 'Column 'Employee.EmpName' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.' on server 4e0652f832fd executing on line 1 (code: 8120, state: 1, class: 16)
Can someone please help me.

Answer 1

select salary
,EmpName
,DeptName
from (
select e.salary
,e.EmpName
,d.DeptName
,rank() over(partition by e.DeptId order by e.salary desc) as rnk
from Employee e join Department d on d.DeptId = e.DeptId
) t
where rnk = 1
salary
EmpName
DeptName
4000
shubh2
ComputerScience
5000
shubh4
Mechanical
12000
shubh5
Aeronautics
1400
shubh7
Civil
Fiddle

Answer 2

Now that I know it's MS SQL Server, technically; we could use cross or outer Apply; it's a table value function not a join per say... but this will depend on the version of SQL Server; and if you want data if it doesn't exist in another
I find this the "Best" Design pattern to use for this type of query.
What the engine does is for each record in department, it runs a query for the employees Finding those in that department returning the 1 record having the max salary. With top we could specify with ties to return more than one. but we would need to know how to handle Ties of salary. Use top 1 with ties or order the results so you get the "Top" result you want.
Demo: dbfddle.uk
SELECT Sub.empName, Sub.Salary, D.DeptName
FROM Department D
CROSS Apply (SELECT Top 1 *
--(SELECT TOP 1 with Ties * -- could use this if we ties
FROM Employee E
WHERE E.DeptID = D.DeptID
ORDER BY Salary Desc) Sub --add additional order by if we don't want ties.
The cross apply gives us:
+---------+--------+-----------------+
| empName | Salary | DeptName |
+---------+--------+-----------------+
| shubh2 | 4000 | ComputerScience |
| shubh4 | 5000 | Mechanical |
| shubh5 | 12000 | Aeronautics |
| shubh7 | 1400 | Civil |
+---------+--------+-----------------+
Before window functions, before cross Apply or lateral... We'd write an inline view
It would get us the max salary for each dept, we then join that back to our base tables to find the employee within each dept with max salary...
Demo: DbFiddle.uk
SELECT E.*, D.*
FROM Employee E
INNER JOIN Department D
on E.DeptID = D.DeptID
INNER JOIN (SELECT MAX(SALARY) maxSal , DeptID
FROM Employee
GROUP BY DeptID) Sub
on Sub.DeptID = E.DeptID
and Sub.MaxSal = E.Salary
One has to do a join to get the department info an the employee info. However, we can eliminate the join for salarymax by using exists and correlation instead.
Demo DbFiddle.uk
SELECT E.*, D.*
FROM Employee E
INNER JOIN Department D
on E.DeptID = D.DeptID
WHERE EXISTS (SELECT MAX(Sub.SALARY) maxSal , Sub.DeptID
FROM Employee Sub
WHERE sub.DeptID=E.DeptID --correlation 1
GROUP BY Sub.DeptID
HAVING E.Salary = max(Sub.Salary)) --correlation 2
We could eliminate the last join too I suppose:
Demo: Dbfiddle.uk
SELECT E.*, (SELECT DeptName from Department where E.DeptID = DeptID)
FROM Employee E
WHERE EXISTS (SELECT MAX(Sub.SALARY) maxSal , Sub.DeptID
FROM Employee Sub
WHERE sub.DeptID=E.DeptID --correlation 1
GROUP BY Sub.DeptID
HAVING E.Salary = max(Sub.Salary)) --correlation 2
The top 3 give us this result:
+-----+---------+--------+--------+--------+-----------------+
| id | empName | salary | deptID | DeptID | DeptName |
+-----+---------+--------+--------+--------+-----------------+
| 101 | shubh2 | 4000 | 1 | 1 | ComputerScience |
| 102 | shubh4 | 5000 | 2 | 2 | Mechanical |
| 103 | shubh5 | 12000 | 3 | 3 | Aeronautics |
| 104 | shubh7 | 1400 | 4 | 4 | Civil |
+-----+---------+--------+--------+--------+-----------------+