1，
SELECT *
FROM work_log_bbs_up
WHERE TYPE IS NULL OR TYPE = 0 OR TYPE = 1 OR TYPE = 2 AND is_del = 0
结果：1166
2，
SELECT *
FROM work_log_bbs_up
WHERE (TYPE IS NULL OR TYPE = 0 OR TYPE = 1 OR TYPE = 2) AND is_del = 0
结果：492
原因：mysql and 优先级比or高，sql 1 会首先计算TYPE = 2 AND is_del = 0然后与其他条件进行or比较。需要使用（）包围起来，明确优先级，因为（）优先级比and高。