SQL Exercise: Details of employees without any department number

Last update on March 31 2023 06:07:16 (UTC/GMT +8 hours)

SQL SORTING and FILTERING on HR Database: Exercise-4 with Solution

4. From the following table, write a SQL query to identify employees who do not have a department number. Return employee_id, first_name, last_name, email, phone_number, hire_date, job_id, salary,commission_pct, manager_id and department_id.

Sample table: employees

Sample Solution:

SELECT *
 FROM employees
  WHERE department_id IS NULL;

Sample Output:

employee_id | first_name | last_name | email | phone_number | hire_date | job_id | salary | commission_pct | manager_id | department_id
-------------+------------+-----------+-------+--------------+-----------+--------+--------+----------------+------------+-------------
(0 rows)

Code Explanation:

The said query in SQL that retrieves all columns from the 'employees' table where the department ID is NULL. The query returns all the rows from the 'employees' table that meet the condition specified in the "WHERE" clause. The "IS NULL" clause is used to check if the value in the "department_id" column is NULL or not.

Relational Algebra Expression:

Relational Algebra Tree:

Practice Online

Query Visualization:

Duration:

Rows:

Cost:

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Previous SQL Exercise: Display Employees whose last name is McEwen.
Next SQL Exercise: Information about the department Marketing.

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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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