SQL - How to optimize the query - sql

Question

I have wrote the following query which extracts all users which are in child departments of current user's department.
Current user cames from client app, but I Decalred it here in SQL for tests reason.
DECLARE #UserID INT = 72
SELECT *
FROM users
WHERE Department_Id IN (
SELECT DISTINCT Id /*, idp*/
FROM departments
WHERE idp IN (
SELECT Department_Id
FROM users
WHERE Id = #UserID
)
)
OR Department_Id IN (
SELECT DISTINCT idp
FROM departments
WHERE idp IN (
SELECT Department_Id
FROM users
WHERE Id = #UserID
)
)
I wanted to select the Id and the idp from departments to have a short query, but when i use this way it returns me an SQL error :
Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.
This is because my list should contain only one column, but not 2.
Please advice me any way to reduce this query, especially the second part (after OR) which is a copy-paste of first (before OR)
Thank you.

Answer 1

Try using EXISTS like this
SELECT *
FROM users u
WHERE EXISTS( SELECT *
FROM departments
WHERE idp IN (SELECT Department_Id FROM users WHERE Id = #UserID)
AND (id = u.Department_Id
OR idp = u.Department_Id) )

Answer 2

A few thoughts...
Nested IN subqueries are unlikely to be friendly.
You don't need DISTINCT when using IN
I'd use GROUP BY to deal with the 1:many relationships, but as your answer is using an alternative structure, I'll try to keep close to what you have...
DECLARE #UserID INT = 72
SELECT
*
FROM
users AS associates
WHERE
EXISTS (
SELECT
*
FROM
users
INNER JOIN
departments
ON departments.idp = users.Department_Id
WHERE
users.id = #user_id
AND ( departments.id = associates.department_id
OR departments.idp = associates.department_id)
)
If you did use the GROUP BY approach, you avoid sub-queries and correlated-sub-queries all together...
DECLARE #UserID INT = 72
SELECT
associates.id
FROM
users
INNER JOIN
departments
ON departments.idp = users.Department_Id
INNER JOIN
users AS associates
ON associates.department_id = departments.id
OR associates.department_id = departments.idp
WHERE
users.id = #user_id
GROUP BY
associates.id
If there any other fields in associates that you need, just add them to the SELECT and the GROUP BY.

Answer 3

SELECT *
FROM users
WHERE Department_Id IN (
SELECT myId FROM
( SELECT Id AS myId
FROM departments
UNION ALL
SELECT idp AS myId
FROM departments
) A
WHERE A.myId IN (
SELECT Department_Id
FROM users
WHERE Id = #UserID
)
)