SQL task: there are two tables A and B with customer data. How to select customers from table A who are not in table B?

To select clients from table A who are not in table B, you can use an SQL query with the NOT IN operator, LEFT JOIN, or NOT EXISTS.

Example with NOT IN:

SELECT *
FROM A
WHERE client_id NOT IN (SELECT client_id FROM B);

Example with LEFT JOIN:

SELECT A.*
FROM A
LEFT JOIN B ON A.client_id = B.client_id
WHERE B.client_id IS NULL;

Example with NOT EXISTS:

SELECT *
FROM A
WHERE NOT EXISTS (
    SELECT 1 FROM B WHERE B.client_id = A.client_id
);

All three options select clients from table A who are not in table B, based on the client_id field (assumed to be the client identifier). The choice of specific option depends on the DBMS and performance preferences.