How can I add multiple rows referring to another table? - mysql

Question

Let's say I have two tables like below:
users table:
user_id
name
0
kevin
1
alice
2
jake
3
mike
permissions table:
user_id
permission
1
12
1
5
3
1
And let's say that I want to add permission 5 to every single user who doesn't already have it. What would be the best MySQL query for this?

Answer 1

Your question is not 100% clear whether you only need a query or you want to do inserts. Anyway, this query with NOT EXISTS can be used as base for all necessary actions:
SELECT user_id, 5 AS permission
FROM users u
WHERE NOT EXISTS
(SELECT 1 FROM permissions
WHERE permission = 5 AND user_id = u.user_id);
This will list all user id's that haven't yet a permission 5 and as second column, just 5 will be selected as permission.
Then this result can be used for whatever it should be used. For example to add this outcome in a query to the already present entries, UNION ALL can be used:
SELECT user_id, 5 AS permission
FROM users u
WHERE NOT EXISTS
(SELECT 1 FROM permissions
WHERE permission = 5 AND user_id = u.user_id)
UNION ALL
SELECT user_id, permission
FROM permissions
ORDER BY user_id, permission;
If - and I think this is your real question - the result of the NOT EXISTS query should be inserted into the permissions table, this insert command will do this:
INSERT INTO permissions
(SELECT user_id, 5
FROM users u
WHERE NOT EXISTS
(SELECT 1
FROM permissions
WHERE permission = 5 AND user_id = u.user_id));
You can try out these things here: db<>fiddle

Answer 2

Below query will look for missing user id in permissions and insert new records in permissions table with a default permission equal to 5.
INSERT INTO permissions
(
user_id,
permission
)
SELECT user_id,
5 AS permission
FROM users
WHERE user_id NOT IN
(
SELECT user_id
FROM permissions)
)
Sample result:
user_id
permission
1
12
1
5
3
1
0
5
2
5