MySQL: Two of every set - php

Question

Assuming I have table "Department" (200 row) & "Employees" (300,000 row) each has their own details
And the vector table combining both is "dep_emp"
CREATE TABLE IF NOT EXISTS `dep_emp` (
`id` int(11) NOT NULL DEFAULT '0',
`dep_id` int(11) NOT NULL DEFAULT '0',
`emp_id` int(11) NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
My question is how to make a query to get two of employees (any two employees) for every Department?
My solution so far is this:
(SELECT * FROM `dep_emp` WHERE dep_id=1 LIMIT 2) UNION ALL
(SELECT * FROM `dep_emp` WHERE dep_id=2 LIMIT 2) UNION ALL
(SELECT * FROM `dep_emp` WHERE dep_id=3 LIMIT 2) ........etc
of course this is done {n} times for each Dept so if I wanted to display two Employees for all dept then I will have to write 199 UNION ALL, this is very ugly
Any other solutions
PS. there are a lot of details that I have moved out of the problem to make things a lot simpler

Answer 1

This may work:
SELECT *
FROM Department
JOIN dep_emp USING (dep_id)
JOIN Employees USING (emp_id)
WHERE emp_id IN
(SELECT emp_id
FROM Employees
JOIN dep_emp USING (emp_id)
WHERE dep_emp.dep_id = Department.dep_id LIMIT 2)
I didn't test it though.
OK, that didn't work. Try this instead:
SELECT *
FROM Department
JOIN dep_emp USING (dep_id)
JOIN Employees USING (emp_id)
JOIN (SELECT emp_id, dep_id
FROM dep_emp
GROUP BY dep_id
LIMIT 2) emp_2
USING (emp_id, dep_id)
BTW don't forget to put some indexes on the tables. On dep_emp put a primary key on both columns AND a unique key on the columns in the reverse order! (That way the index works either way.) (You don't need the ID column in dep_emp BTW, at least not in the structure you showed.)

Answer 2

SELECT d.*, e.*
FROM Department AS d
JOIN Employees AS e
ON e.id IN
( SELECT de.emp_id
FROM dep_emp AS de
WHERE de.dep_id = d.id
LIMIT 2
)
Another, very different way to get two emp_id for every department:
SELECT dep_id
, MIN(emp_id) AS emp_id_1
, MAX(emp_id) AS emp_id_2
FROM dep_emp
GROUP BY dep_id
or (to have in separate rows, so this can be joined to the other 2 tables):
SELECT dep_id
, MIN(emp_id) AS emp_id
FROM dep_emp
GROUP BY dep_id
UNION ALL
SELECT dep_id
, MAX(emp_id)
FROM dep_emp
GROUP BY dep_id
The first query does not work in MySQL, because LIMIT is not allowed inside IN subqueries. Here's another approach:
SELECT e.*
FROM
( SELECT DISTINCT dep_id
FROM dep_emp
) AS d
JOIN
dep_emp AS e
ON d.dep_id = e.dep_id
AND e.emp_id <=
COALESCE( ( SELECT de.emp_id
FROM dep_emp AS de
WHERE de.dep_id = d.dep_id
ORDER BY de.emp_id --- OFFSET 1
LIMIT 1 OFFSET 1 --- to get 2 employess
) --- per department
, 9999999 )
ORDER BY e.dep_id
, e.emp_id
Use OFFSET x to get x+1 employees per department.