SQL Exercise: Jobs done by two or more for more than 300 days

Last update on March 31 2023 06:07:48 (UTC/GMT +8 hours)

SQL SORTING and FILTERING on HR Database: Exercise-24 with Solution

24. From the following table, write a SQL query to find each job ids where two or more employees worked for more than 300 days. Return job id.

Sample table : job_history

Sample Solution:

SELECT job_id 
	FROM job_history 
		WHERE end_date-start_date >300 
			GROUP BY job_id 
				HAVING COUNT(*)>=2;

Sample Output:

   job_id
------------
 AC_ACCOUNT
 ST_CLERK
(2 rows)

Code Explanation:

The said query in SQL that retrieves job_id values from the 'job_history' table where the duration of a job (calculated as end_date - start_date) is greater than 300 days. The query then groups the results by job_id and returns only those groups that have at least two records (HAVING COUNT(*) >= 2).
The final result of this calculation will result in a list of the job_ids that have been held by at least two employees for more than 300 days.

Relational Algebra Expression:

Relational Algebra Tree:

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Query Visualization:

Duration:

Rows:

Cost:

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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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