SQL Exercise: The date when he left his last job and his employee ID

Last update on March 31 2023 06:07:03 (UTC/GMT +8 hours)

SQL SORTING and FILTERING on HR Database: Exercise-32 with Solution

32. From the following table, write a SQL query to find those employees who have completed their previous jobs. Return employee ID, end_date.

Sample table : job_history

Sample Solution:

SELECT employee_id , MAX(end_date)
FROM job_history
WHERE employee_id IN (SELECT employee_id
FROM job_history
GROUP BY 1
HAVING COUNT(employee_id) > 1)
GROUP BY 1

Sample Output:

 employee_id |    max
-------------+------------
         101 | 2005-03-15
         200 | 2006-12-31
         176 | 2007-12-31
(7 rows)

Code Explanation:

The said query in SQL that retrieves the employee ID and the latest end date for each employee with multiple job history entries. The query first selects the employee IDs that have more than one entry in the 'job_history' table using a subquery and the "HAVING COUNT" clause. Then, it retrieves the employee IDs and their corresponding maximum end dates from the 'job_history' table, grouping the results by the employee IDs.

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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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