SQL Exercise: Departments with managers manage more than 4 employees
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.
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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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