Select in another MySQL table

To bring only the customer data, based on a user ID (since you have the user_id user reference in the customer table), you can do this:

SELECT * FROM clients
WHERE user_id = 123456

But if you want to bring client data AND user data, add the join in the query.

SELECT * FROM clients
JOIN users ON users.id = clients.user_id
WHERE clients.user_id = 123456

By joining you will have the list of all the names of the clients table that are in the users table.

SELECT clients.name 
FROM users INNER JOIN clients 
ON (users.id = clients.user_id)

If you want to bring all the columns, just replace name with *

SELECT clients.* 
FROM users INNER JOIN clients 
ON (users.id = clients.user_id)