SQL Exercise: Find employees managed by the manager

Last update on March 31 2023 06:07:47 (UTC/GMT +8 hours)

SQL SORTING and FILTERING on HR Database: Exercise-26 with Solution

26. From the following table, write a SQL query to count the number of employees worked under each manager. Return manager ID and number of employees.

Sample table: employees

Sample Solution:

SELECT manager_id, COUNT(*) 
	FROM employees 
		GROUP BY manager_id;

Sample Output:

 manager_id | count
------------+-------
        205 |     1
        122 |     8
        120 |     8
        101 |     5
        103 |     4
        108 |     5
        145 |     6
        100 |    14
        201 |     1
        124 |     8
        114 |     5
        121 |     8
        123 |     8
        102 |     1
        146 |     6
        147 |     6
        148 |     6
        149 |     6
          0 |     1
(19 rows)

Code Explanation:

The said query in SQL that returns the number of employees for each manager in the 'employees' table. It does so by grouping the rows in the table by the value in the "manager_id" column, then counting the number of rows in each group.

Relational Algebra Expression:

Relational Algebra Tree:

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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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