ALTER TABLE `job_posting`
    ADD `has_referrer` BIT(1) NOT NULL DEFAULT b'0',
    ADD INDEX (`has_referrer`);
ALTER TABLE `candidate_basic_info`
    ADD `referrer` BIGINT NULL DEFAULT NULL,
    ADD INDEX (`referrer`);
ALTER TABLE `employee_history`
    ADD `referrer` BIGINT NULL DEFAULT NULL,
    ADD INDEX (`referrer`);