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