这个sql是什么意思,求解

请问下面两个sql语句有什么区别,
我按照数据库写的下面的那个,并没有读懂第一个sql语句
有必要去学第一种吗?

insert into @tmp select Id,@Layer from MIX_单位 where Id in (
    select OrgId from MIX_UserPrivilege where UserId='BE9EE1CC-1818-43FF-8F1E-02098A8F59ED'
) 
while @@ROWCOUNT>0 
begin 
    select @Layer=@Layer+1 
    insert into @tmp select Id,@Layer from MIX_单位 where ParentId in (
        select Id from @tmp where Layer=@Layer-1 
    ) and Id not in (
        select Id from @tmp 
    ) 
end 
select Id from MIX_机组 where 单位Id in (
    select Id from @tmp 
) 

select Id from MIX_机组 where 单位Id in( select Id from MIX_单位 where ParentId in( select OrgId from MIX_UserPrivilege))