SQL Exercise: Find those employees hired before June 21st, 2002

Last update on March 31 2023 06:07:11 (UTC/GMT +8 hours)

SQL SORTING and FILTERING on HR Database: Exercise-15 with Solution

15. From the following table, write a SQL query to find the employees who were hired before June 21st, 2002. Return all fields.

Sample table: employees

Sample Solution:

SELECT *
 FROM employees
  WHERE hire_date < '2002-06-21';

Sample Output:

employee_id | first_name | last_name |  email   | phone_number | hire_date  |   job_id   |  salary  | commission_pct | manager_id | department_id
-------------+------------+-----------+----------+--------------+------------+------------+----------+----------------+------------+---------------
         102 | Lex        | De Haan   | LDEHAAN  | 515.123.4569 | 2001-01-13 | AD_VP      | 17000.00 |           0.00 |        100 |            90
         203 | Susan      | Mavris    | SMAVRIS  | 515.123.7777 | 2002-06-07 | HR_REP     |  6500.00 |           0.00 |        101 |            40
         204 | Hermann    | Baer      | HBAER    | 515.123.8888 | 2002-06-07 | PR_REP     | 10000.00 |           0.00 |        101 |            70
         205 | Shelley    | Higgins   | SHIGGINS | 515.123.8080 | 2002-06-07 | AC_MGR     | 12000.00 |           0.00 |        101 |           110
         206 | William    | Gietz     | WGIETZ   | 515.123.8181 | 2002-06-07 | AC_ACCOUNT |  8300.00 |           0.00 |        205 |           110
(5 rows)

Code Explanation:

The said query in SQL that retrieves all columns (*) from the 'employees' table where the value in the "hire_date" column is earlier than the date '2002-06-21'.

Relational Algebra Expression:

Relational Algebra Tree:

Practice Online

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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