简单查询树形结构数据库表

/****创建自定义函数****/
CREATE FUNCTION dbo.fGetTreeTable
(
@ID int= null
)
RETURNS @Tab TABLE(ID int, PID int, Name varchar(10), Lev int)--返回结果的表结构
AS
BEGIN
    Declare @lev int
    Set @lev=0
    
    While @lev=0 or @@ROWCount>0
    Begin
     Set @Lev=@Lev+1
     Insert @Tab(ID, PID, Name, Lev)
     Select ID, PID, Name, @Lev From Tree Where (@Lev=1 and ((PID=@ID) or (@ID is null and PID is null))) or (PID in (Select ID From @Tab Where Lev=@Lev-1))
     order by ID
    End
    RETURN
END 
直接查询Select * from dbo.fGetTreeTable(null)就可以输入所有记录。