為什么mysql中g(shù)roup by時候查詢數(shù)據(jù)為空是count為null，不為0吶

為什么mysql中g(shù)roup by時候查詢數(shù)據(jù)為空是count為null，不為0吶
版本5.6.11，
sql：

select  count(lat.CABINET_NO) reserve
from lattice_info lat,cabinet_info cab where lat.CABINET_NO = cab.CABINET_NO and 
     lat.CUSTOMER_CODE=cab.CUSTOMER_CODE and lat.SUB_ACCOUNT = '00' and 
     lat.MODIFY_TIME >= '2017-11-12' and lat.MODIFY_TIME <= '2017-11-15' GROUP BY cab.cabinet_no

這種時候查詢的結(jié)果集為空，count為null

select  count(lat.CABINET_NO) reserve
from lattice_info lat,cabinet_info cab where lat.CABINET_NO = cab.CABINET_NO and 
     lat.CUSTOMER_CODE=cab.CUSTOMER_CODE and lat.SUB_ACCOUNT = '00' and 
     lat.MODIFY_TIME >= '2017-11-12' and lat.MODIFY_TIME <= '2017-11-15' 

去掉group by， count就為0