mysql having的意义

SELECT  id, (3959 * acos(cos ( radians(78.3232) )  * cos( radians( lat ) )  * cos( radians( lng ) - radians(65.3234) )  + sin ( radians(78.3232) )  * sin( radians( lat ) )  )  ) AS distance  
    FROM markers  
    HAVING distance < 30  
    ORDER BY distance  
    LIMIT 0 , 20;