SQL Exercise: Find employees who did two or more jobs in the past
SQL SORTING and FILTERING on HR Database: Exercise-22 with Solution
22. From the following table, write a SQL query to find those employees who worked more than two jobs in the past. Return employee id.
Sample table : job_history
Sample Solution:
SELECT employee_id
FROM job_history
GROUP BY employee_id
HAVING COUNT(*) >=2;
Sample Output:
employee_id ------------- 101 176 200 (3 rows)
Code Explanation:
The said query in SQL that is selecting the "employee_id" from the 'job_history' table, grouping the results by the "employee_id" column, and only returning the groups that have a count of 2 or more records. In other words, it returns the "employee_id" values of employees who have changed jobs at least twice based on the records in the "job_history" table.
Relational Algebra Expression:
Relational Algebra Tree:
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Previous SQL Exercise: Find employees who is working given departments.
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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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