SELECT * FROM `table` WHERE date(時間字段) = curdate();

SELECT *
FROM `table`
WHERE month( 時間字段) = month( now( ) ) ;

SELECT *
FROM `table`
WHERE month( from_unixtime( reg_time ) ) = month( now( ) ) ;

SELECT *
FROM `table`
WHERE month( from_unixtime( reg_time ) ) = month( now( ) ) -1;