ALTER TABLE mp_visitor
    ADD COLUMN is_default TINYINT NOT NULL DEFAULT 0 COMMENT '是否默认来访人' AFTER gender,
    ADD KEY idx_mp_visitor_user_default (mp_user_id, is_default, del_flag);

UPDATE mp_visitor v
JOIN (
    SELECT MIN(id) AS id
    FROM mp_visitor
    WHERE del_flag = 0
    GROUP BY mp_user_id
) first_visitor ON first_visitor.id = v.id
SET v.is_default = 1
WHERE v.del_flag = 0;