alexanderzanin

CREATE TABLE client_balance
(
    id SMALLINT AUTO_INCREMENT,
    id_client INTEGER,
    balance_date_start DATETIME,
    balance_start INTEGER,
    balance_date_mid DATETIME,
    balance_mid INTEGER,
    PRIMARY KEY (id)
);

SELECT cb.*,
       DATEDIFF(cb.balance_date_mid, cb.balance_date_start) AS diff
FROM client_balance AS cb
GROUP BY cb.id_client
HAVING MAX(diff);