Setting a column value for only one user depending on results - sql

Question

I'm currently playing around with SQL and trying to find the best way to accomplish this:
I currently have a user table that has a user_id, organisation_id, registered_datetime. There are
a number of users in this table with different organisations. There may be 3 different users in
1 organisation, or 1 in 1 organisation, etc.
I have added a new column called admin_user and I am trying to string up an SQL statement together
to update the admin user column. There can only be one admin user per organisation, and I want
the user who registered the earliest for that organisation to be the admin.
I could do this manually but it would take time if I had a lot of users. What would be the best
way to accomplish this?
EDIT:
So I have a number of users like this with the columns. The ones highlighted are the users that has registered the earliest. I want to be able to set those users as an admin user. The only admin user within their organisation and set the rest to 0. 1 (Admin) 0 (Not Admin)

Answer 1

This SQL query will mark users which registered_datetime are lowest in its organisation_id as admin.
UPDATE users SET admin_user = 1
WHERE user_id IN (
SELECT u.user_id FROM users u
WHERE u.registered_datetime IS NOT NULL AND NOT EXISTS(
SELECT 1 FROM users iu
WHERE iu.organisation_id = u.organisation_id AND iu.registered_datetime < u.registered_datetime
)
)
You might want to update all users to admin_user = 0 before this code, so all your users will have their values set.
One caveat here, if two users in one organisation were registered in exact same time, then both of them will be marked as administrators.
Update
I have added u.registered_datetime IS NOT NULL into the WHERE clause to filter out users with NULL in registered_datetime.
MSSQL
In MsSql server I usually solve this problem a in another way, by using ROW_NUMBER():
WITH base AS (
SELECT user_id, ROW_NUMBER() OVER ( PARTITION BY organisation_id ORDER BY registered_datetime ASC ) AS rn
FROM user
WHERE registered_datetime IS NOT NULL
)
UPDATE user SET is_admin = 1
WHERE user_id IN (
SELECT base.user_id FROM base WHERE rn = 1
)

Answer 2

This is too long for a comment.
You are describing three different tables:
Users
Organizations
UserOrganizations
The last has one row per user and per organization. This provides the mapping between the two. This can be called a "mapping" table, "junction" table, or "association" table.
How you implement one admin per organization depends on the database you are using.

Answer 3

You do not need the admin_user column. You need a column isadmin.
When a user is registered, if he is the first in the organization, then the isadmin column has the value 1, otherwise 0
Also, you can use the AAA query to find the administrator
SELECT `table`.`user_id`, MIN(`table`.`registered_datetime`) WHERE `organisation_id`=...