SQL Exercise: Departments with managers manage more than 4 employees

Last update on March 31 2023 06:07:45 (UTC/GMT +8 hours)

SQL SORTING and FILTERING on HR Database: Exercise-30 with Solution

30. From the following table, write a SQL query to find the departments where any manager manages four or more employees. Return department_id.

Sample table : employees

Sample Solution:

SELECT DISTINCT department_id
	FROM employees
		GROUP BY department_id, manager_id 
			HAVING COUNT(employee_id) >=4;

Sample Output:

 department_id
---------------
            80
            50
            60
           100
            30
(5 rows)

Code Explanation:

The said query in SQL that returns a list of unique department IDs from the 'employees' table where there are at least 4 employees with the same manager in each department.
The query starts by selecting only distinct "department_id" values from the 'employees' table, then groups the results by "department_id" and "manager_id".
The HAVING clause is used to specify a condition on the grouped result, in this case counting the number of "employee_id"s in each group and returning only those groups where the count is greater than or equal to 4.

Relational Algebra Expression:

Relational Algebra Tree:

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Previous SQL Exercise: Average salary of each commission-based department.
Next SQL Exercise: Departments more than 10 employees who get commissions.

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SQL: Tips of the Day

Difference between natural join and inner join

One significant difference between INNER JOIN and NATURAL JOIN is the number of columns returned-

Consider:

TableA                           TableB
+------------+----------+        +--------------------+    
|Column1     | Column2  |        |Column1  |  Column3 |
+-----------------------+        +--------------------+
| 1          |  2       |        | 1       |   3      |
+------------+----------+        +---------+----------+

The INNER JOIN of TableA and TableB on Column1 will return

SELECT * FROM TableA AS a INNER JOIN TableB AS b USING (Column1);
SELECT * FROM TableA AS a INNER JOIN TableB AS b ON a.Column1 = b.Column1;

+------------+-----------+---------------------+    
| a.Column1  | a.Column2 | b.Column1| b.Column3|
+------------------------+---------------------+
| 1          |  2        | 1        |   3      |
+------------+-----------+----------+----------+

The NATURAL JOIN of TableA and TableB on Column1 will return:

SELECT * FROM TableA NATURAL JOIN TableB
+------------+----------+----------+    
|Column1     | Column2  | Column3  |
+-----------------------+----------+
| 1          |  2       |   3      |
+------------+----------+----------+

Ref: https://bit.ly/3AG5CId

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