Help me with this SQL Query - sql

Question

3 tables are defined as follows:
Users
User_ID INT
First_Name VARCHAR
Last_Name VARCHAR
Email VARCHAR
Roles
Role_ID INT
Role_Name VARCHAR
Access_Level INT
Roles_Users
User_ID INT
Role_ID INT
Roles_Users is a many-to-many linking table between Users and Roles. I want to pull back the following info:
First_Name, Last_Name, Email, Role_Name
What I have so far is:
SELECT
U.First_Name,
U.Last_Name,
U.Email,
R.Name AS Role_Name
FROM Users U
INNER JOIN Roles_Users RU ON U.User_ID = RU.User_ID
INNER JOIN Roles R ON RU.Role_ID = R.Role_ID
The tricky part (at least for me) is that I want to only pull back the Role_Name with the MIN(Access_Level) for that particular user. So basically the record set I want to pull will have each user only listed once with their lowest access level role name.
I'm sure this is pretty simple but it's just stumping me right now.
Thanks

Answer 1

YOu can use a CTE (Common Table Expression) in conjunction with the ROW_NUMBER windowing function like this:
;WITH MinAccessData AS
(
SELECT
U.First_Name,
U.Last_Name,
U.Email,
R.Name AS Role_Name,
ROW_NUMBER() OVER(PARTITION BY U.User_ID ORDER BY R.Access_Level) AS RowNum
FROM Users U
INNER JOIN Roles_Users RU ON U.User_ID = RU.User_ID
INNER JOIN Roles R ON RU.Role_ID = R.Role_ID
)
SELECT *
FROM MinAccessData
WHERE RowNum = 1
The CTE "partitions" your data by User_ID, e.g. each user gets a "partition". Inside that partition, the roles are ordered by Access_level and the smallest is the first one - so it gets RowNum = 1 - for each user.
So you then select from that CTE all those entries where the RowNum = 1 - this delivers all the entries for each user which have the smallest Access_Level value.

Answer 2

Alternatives without a CTE (just to have another tool in your box)
SELECT
U.First_Name,
U.Last_Name,
U.Email,
R.Name AS Role_Name
FROM Users U
INNER JOIN Roles_Users RU ON U.User_ID = RU.User_ID
INNER JOIN Roles R ON RU.Role_ID = R.Role_ID
INNER JOIN (SELECT
MIN(r.Access_Level) access_level,
ru.UserID,
FROM Roles r
INNER JOIN Roles_Users ru
ON r.Role_ID = ru.Role_ID
GROUP BY UserID
) minAccess
ON ru.UserId = minAccess.UserId
and ru.
ON r.access_level = minAccess .access_level
You can also use a CROSS APPLY
SELECT
U.First_Name,
U.Last_Name,
U.Email,
R.Name AS Role_Name
FROM Users U
CROSS APPLY (SELECT TOP 1
Role_Name
FROM Roles_Users RU ON U.User_ID = RU.User_ID
INNER JOIN Roles R ON RU.Role_ID = R.Role_ID
WHERE u.user_id = ru.user_id
ORDER BY
Access_Level desc
)

Answer 3

Correlated subquery:
SELECT
U.First_Name,
U.Last_Name,
U.Email,
(
select top 1 R.RName from Roles_Users RU ON U.User_ID = RU.User_ID
INNER JOIN Roles R ON RU.Role_ID = R.Role_ID
ORDER BY R.Access_Level
)
AS Role_Name
FROM Users U
In my opinion using a subquery is easier to read and write. In this code the correlated subquery will execute 1x per row returned. I like #Conrad's inner join solution, easiest and probably the most performant, and probably what i would use, just giving this as another option.

Answer 4

Not tested, but it goes something like this
SELECT
U.First_Name,
U.Last_Name,
U.Email,
R.Role_Name
FROM Users U
JOIN Roles_Users RU ON U.User_ID = RU.User_ID
JOIN (
SELECT ROLE_ID, MIN(ROLE_NAME) ROLE_NAME
FROM ROLES
GROUP BY ROLE_ID
HAVING ACCESS_LEVEL = MIN(ACCESS_LEVEL)
) R ON RU.Role_ID = R.Role_ID

Answer 5

SELECT Users.*, Roles.*
FROM
Users
JOIN Roles_Users ON Users.User_ID = Roles_Users.User_ID
JOIN Roles ON Roles.Role_ID = Roles_Users.Role_ID
WHERE
Access_Level = (
SELECT MIN(Access_Level)
FROM
Roles_Users
JOIN Roles ON Roles.Role_ID = Roles_Users.Role_ID
WHERE Users.User_ID = Roles_Users.User_ID
)
NOTE: This will not list users without any role.