SQL循环添加表中的字段

USE BookDiscuss
                 
DECLARE @num INT 
DECLARE @numdiff INT 
DECLARE @table  VARCHAR(50)
DECLARE @sql VARCHAR(8000)
SET @numdiff=3
SET @num=0 
WHILE @num<@numdiff 
BEGIN
SET @table='DiscussPost_'+CAST(@num AS VARCHAR(10));
SET    @sql=
'ALTER TABLE '+@table+' ADD  PostFatherId INT  DEFAULT 0;
ALTER TABLE '+@table+' ADD PostReplyUserId INT DEFAULT 0;'
EXEC(@sql)
SET @num=@num+1 
END