请问下面2个查询语句有什么区别，为什么获取到的记录集不一样？

请问下面2个查询语句有什么区别，为什么获取到的记录集不一样？
1.
select t1.telno from users t1
where exists (
select * from log t2
where t1.telno=substr(ferr,49,11)
and t1.code=substr(ferr,78,4)
    and t2.errs='数字错误'
    and t2.name='nailor'
)2.
select t1.telno from users t1,log t2
where t1.telno=substr(t2.ferr,49,11)
and t1.code=substr(t2.ferr,78,4)
    and t2.errs='数字错误'
    and t2.name='nailor'