Write a solution to show the unique ID of each user, If a user does not have a unique ID replace just show null.

Assuming you are using SQL, here is a step-by-step solution:

1. First, you need to select the user ID from your user table. You can do this with the following command:

SELECT user_id FROM users;

2. Next, you need to check if the user ID is unique. You can do this by using the DISTINCT keyword, which will remove duplicate entries:

SELECT DISTINCT user_id FROM users;

3. If a user does not have a unique ID, you want to show null. You can do this by using the COALESCE function, which returns the first non-null value in a list. If all values are null, it returns null:

SELECT COALESCE(user_id, 'null') FROM users;

4. Finally, you want to combine these steps into one command:

SELECT COALESCE(DISTINCT user_id, 'null') FROM users;

Please note that this command will not work in all SQL versions because some do not allow the use of DISTINCT inside COALESCE. In that case, you would need to use a subquery or a temporary table.