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

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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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