请高手解释这段代码的原理

USE [Northwind]
GO
/****** 对象: StoredProcedure [dbo].[USP_TREEVIEW] 脚本日期: 12/18/2009 07:59:28 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[USP_TREEVIEW]
@TABLE NVARCHAR(50) --表名
,@PIDCOL NVARCHAR(50) --父结点标识字段名
,@IDCOL NVARCHAR(50) --子结点标识字段名
,@DESCCOL NVARCHAR(50) --显示节点标签字段
AS
BEGIN
DECLARE @SQL VARCHAR(8000)

SELECT @SQL='
DECLARE @TMP TABLE(
[PID_NUM] [INT] NULL,
[ID_NUM] [int] IDENTITY (1, 1) NOT NULL ,
['+@PIDCOL+'] [nvarchar] (20) NULL ,
['+@IDCOL+'] [nvarchar] (20) NOT NULL ,
['+@DESCCOL+'] [ntext] NULL,
PATH NVARCHAR(1000) NULL,
NEXT_PARENTID INT NULL,
PRE_PARENTID INT NULL,
PATHCHAR NVARCHAR(1000) DEFAULT('''') NULL,
ISFIRST BIT DEFAULT(1) NULL
)
INSERT @TMP (['+@PIDCOL+'],['+@IDCOL+'],['+@DESCCOL+'])
SELECT ['+@PIDCOL+'],['+@IDCOL+'],['+@DESCCOL+'] FROM '+@TABLE+' order by '+@PIDCOL+','+@IDCOL+'

UPDATE A SET PID_NUM=B.ID_NUM,PATH=CAST(A.ID_NUM AS NVARCHAR),NEXT_PARENTID=B.ID_NUM
FROM @TMP A LEFT JOIN @TMP B ON A.'+@PIDCOL+'=B.'+@IDCOL+'

WHILE @@ROWCOUNT!=0
UPDATE A SET
A.PATH=ISNULL(CAST(A.NEXT_PARENTID AS NVARCHAR)+''.'','''')+A.PATH
,A.PATHCHAR=CASE WHEN A.ISFIRST=1 THEN
CASE WHEN not exists (select * from @TMP where PID_NUM=A.NEXT_PARENTID and ID_NUM>A.ID_NUM ) THEN
''└'' + replicate(''─'', 2) + A.PATHCHAR
ELSE
''├'' +replicate(''─'', 2) + A.PATHCHAR
END
ELSE
CASE WHEN exists (select * from @TMP where PID_NUM=A.NEXT_PARENTID and ID_NUM>( select ID_NUM from @TMP where ID_NUM=A.PRE_PARENTID) ) THEN
''│'' + replicate(''　'', 2) +A.PATHCHAR
ELSE
replicate(''　'', 3) +A.PATHCHAR
END
END
,A.PRE_PARENTID=A.NEXT_PARENTID
,A.NEXT_PARENTID=B.PID_NUM
,A.ISFIRST=0
FROM @TMP A LEFT JOIN @TMP B ON A.NEXT_PARENTID=B.ID_NUM
WHERE A.NEXT_PARENTID IS NOT NULL
--SELECT * FROM @TMP ORDER BY PATH
SELECT PATHCHAR+CAST(['+@DESCCOL+'] AS NVARCHAR) AS '+'[Label_'+@DESCCOL+'],['+@IDCOL+'],['+@PIDCOL+']
FROM @TMP
ORDER BY PATH
'
--print (@SQL)
exec( @SQL)
END
EXEC USP_TREEVIEW 'Northwind..Employees','ReportsTo','EmployeeID','LastName'