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mysql获取当前时间戳为UNIX_TIMESTAMP(NOW()),即SELECT UNIX_TIMESTAMP(NOW());,运行结果：1553221026

mysql获取当天日期用CURDATE()，即SELECT CURDATE();，运行结果2019-03-22

mysql获取当天日期时间戳为UNIX_TIMESTAMP(CURDATE()),即SELECT UNIX_TIMESTAMP(CURDATE());,运行结果：1553184000