Display ID sharing 2 values from same attribute - mysql

Question

I am trying to get the eNum (employee number) of that who masters 2 values (MySQL and Python) from the same attribute column, The closest I get is down below, but the eNum is duplicated. I want to get just one eNum once. I think I am messing it up in the WHERE clause... I don't know...
mysql> select * from employee_expert;
+------+---------+
| eNum | package |
+------+---------+
| E246 | Excel |
| E246 | MySQL |
| E246 | Python |
| E246 | Word |
| E403 | Jave |
| E403 | MySQL |
| E892 | Excel |
| E892 | PHP |
| E892 | Python |
+------+---------+
mysql> SELECT eNum, package
FROM employee_expert
WHERE (package = 'MySQL' OR package = 'Python') AND (package = 'MySQL' OR package = 'Python')
GROUP BY package;
+------+---------+
| eNum | package |
+------+---------+
| E246 | MySQL |
| E246 | Python |
+------+---------+

Answer 1

The WHERE clauses contains unnecessary duplication of the condition package = 'MySQL' OR package = 'Python'. Using WHERE (package = 'MySQL' OR package = 'Python') is enough. Or, to make it more readable you can write WHERE package IN ('MySQL', 'Python').
Your query selects the employees that know 'MySQL' or 'Python' or both.
It looks like you want to select the employees that know both 'MySQL' and 'Python'. You need to use a JOIN for this purpose:
SELECT f.eNum
FROM employee_expert f # 'f' from 'first'
INNER JOIN employee_expert s USING(eNum) # 's' from 'second'
WHERE f.package = 'MySQL'
AND s.package = 'Python'
Unfortunately, this approach does not scale very well if you need to find by a larger set of languages. A better approach would be to use the original query and group the results by eNum like this:
SELECT eNum, COUNT(DISTINCT package) AS nbLangs
FROM employee_expert
WHERE package IN ('MySQL', 'Python') # <------------------------------------+
GROUP BY eNum # Make one entry for each employee |
HAVING nbLangs = 2 # Replace '2' with the number of items in this list --+
This query counts the number of known languages for all the employees that know at least one of the languages in the list then keeps only those that knows all of them.

Answer 2

I think the problem is in the design itself, come to think of it, an employee can master MANY packages and a package can be mastered by MANY employees, it's a many to many relationship, in terms of database that will produce a table employee_package for example which contains a primary key composed of the primary key of each table
+------+------------+
| eNum | package_id |
+------+------------+
| E246 | 1 |
| E246 | 2 |
| E246 | 3 |
| E892 | 1 |
+------+------------+
then your request will be something like :
SELECT DISTINCT e.eNum from employees e JOIN employee_package ep on ep.eNum = e.eNum
WHERE ep.package_id = 1 OR ep.package_id = 2
-- let's say that id 1 is for MySQL and id 2 is for Python