﻿ SQL: Find employees who did two or more jobs in the past

# 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:

## Query Visualization:

Duration:

Rows:

Cost:

Previous SQL Exercise: Find employees who is working given departments.
Next SQL Exercise: Difference between highest and lowest salary for a job.

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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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