跨库修改

UPDATE scrm.t_contacts t
SET t.MAIN_USER_ID = (
    SELECT

    IF (
        u.`PARENT_ID` = '0',
        u.`ID`,
        u.`PARENT_ID`
    ) MAIN_USER_ID
    FROM
        `user-center`.tb_user u
    WHERE
        t.USER_ID = u.`ID`
);