内连接自连接和内嵌自连接的区别

 select (SELECT count(1)FROM  entity.PhysicalItemResult sub,entity.PhysicalItemResult main
WHERE sub.physicalitemid = main.physicalitemid  AND sub.physicalitemresultid <= main.physicalitemresultid
) as '序号',physicalitemid,physicalitemresultid
from entity.PhysicalItemResult
-------------------------------
SELECT(SELECT COUNT(1) FROM entity.PhysicalItemResult sub WHERE sub.physicalitemid = main.physicalitemid
AND sub.physicalitemresultid <= main.physicalitemresultid) AS 序号,physicalitemid,physicalitemresultid
FROM entity.PhysicalItemResult main

为什么第二次数据是这样？内嵌，

每一对main的physicalitemid，physicalitemresultid就对应一次嵌套查询（循环嵌套？）