How to display the identifiers of clients who do not have an account in one SQL query?

Question

MySQL

It is necessary to write an SQL query that displays the identifiers of clients who do not have an account. As well as the identifiers of those clients that are recorded in the Dep table, but are not in the Contact table. As a result, you need to get a list of identifiers.

Answer 1

SELECT client
FROM ( SELECT DISTINCT client
       FROM Dep
     UNION ALL
       SELECT client_id
       FROM Contact ) total
GROUP BY 1
HAVING COUNT(*) = 1