SQL Employees Searching for Others in Same Department - sql

Question

I have a business rule that employees cannot purchase items from employees of the same department. I have two tables. one is the list of employees and their IDs:
Emp_ID Emp_Name Dept_ID
1 John 1
2 Bob 1
3 Susie 2
4 Jack 3
5 Jill 3
And a table of the employee ID and the employee ID they purchased from:
Emp_ID Bought_From_Emp_ID
1 2
2 3
4 5
5 1
My expected output would be to have the the employee id (or name) of both employees if one purchased an item from the same department:
Emp_ID Bought_From_Emp_ID Same_Dept_ID
1 2 1 --John and Bob are in Same Department (1)
4 5 3 --Jack and Jill are in Same Department (3)
How would I do this for millions of records? I have a feeling that this is very simple in the long run, but my mind hasn't shifted towards the solution yet.
I am using Teradata, but can use MSSQL if there are any SQL-specific answers

Answer 1

DECLARE #emp TABLE
(
emp_id INT,
emp_name VARCHAR(20),
dept_id INT
);
INSERT INTO #emp
(
emp_id,
emp_name,
dept_id
)
VALUES
--Emp_ID Emp_Name Dept_ID
(1, 'John ', 1),
(2, 'Bob', 1),
(3, 'Susie', 2),
(4, 'Jack', 3),
(5, 'Jill', 3);
DECLARE #purch TABLE
(
emp_id INT,
Bought_From_Emp_ID INT
);
INSERT INTO #purch
(
emp_id,
Bought_From_Emp_ID
)
VALUES
(1, 2),
(2, 3),
(4, 5),
(5, 1);
SELECT e.emp_id,
e1.emp_id AS Bought_From_Emp_ID,
e.dept_id AS Same_Dept_ID
FROM #purch p
JOIN #emp e
ON p.emp_id = e.emp_id
JOIN #emp e1
ON p.Bought_From_Emp_ID = e1.emp_id
AND e.dept_id = e1.dept_id
WHERE e1.emp_id <> e.emp_id;

Answer 2

Try this query
SELECT purch.emp_id AS EmpID,
purch.bought_from_emp_id AS BoughtFrom,
T1.dept_id AS department
FROM purch
INNER JOIN emp T1
ON T1.emp_id = purch.emp_id
INNER JOIN emp T2
ON T2.emp_id = purch.bought_from_emp_id
WHERE t1.dept_id = t2.dept_id
Output
+-------+------------+------------+
| EmpID | BoughtFrom | department |
+-------+------------+------------+
| 1 | 2 | 1 |
| 4 | 5 | 3 |
+-------+------------+------------+
Demo: http://www.sqlfiddle.com/#!18/22746/1/0