Orcale数据库sum()和count()的区别

具体SQL如下，表名table,查询列名Age varchar2类型 有什么区别呢 数据量百万时哪个运行效率快一点

select

count(decode(t.Age,'20',1,null) ) as Age等于20数量count,

sum(decode(t.Age,'20',1,null)) as Age等于20数量sum,

count(case when t.Age='20' then 1 else 0 end) as Age等于20的数量casecount,

sum(case when t.Age='20' then 1 else 0 end) as Age等于20的数量casesum

from table t;