解析如何用SQL语句在指定字段前面插入新的字段

create proc addcolumn
@tablename varchar(30), –表名
@colname varchar(30), –要加的列名
@coltype varchar(100), –要加的列类型
@colid int –加到第几列
as

declare @colid_max int
declare @sql varchar(1000) –动态sql语句
————————————————–
if not exists(select 1 from sysobjects
where name = @tablename and xtype = ‘u’)
begin
raiserror 20001 ‘没有这个表’
return -1
end
————————————————–
if exists(select 1 from syscolumns
where id = object_id(@tablename) and name = @colname)
begin
raiserror 20002 ‘这个表已经有这个列了！’
return -1
end
————————————————–
–保证该表的colid是连续的
select @colid_max = max(colid) from syscolumns where id=object_id(@tablename)

if @colid > @colid_max or @colid < 1
set @colid = @colid + 1
————————————————–
set @sql = ‘alter table ‘+@tablename+’ add ‘+@colname+’ ‘+@coltype
exec(@sql)

select @colid_max = colid
from syscolumns where id = object_id(@tablename) and name = @colname
if @@rowcount <> 1
begin
raiserror 20003 ‘加一个新列不成功，请检查你的列类型是否正确’
return -1
end
————————————————–
–打开修改系统表的开关
EXEC sp_configure ‘allow updates’,1 RECONFIGURE WITH OVERRIDE

–将新列列号暂置为-1
set @sql = ‘update syscolumns
set colid = -1
where id = object_id(”’+@tablename+”’)
and colid = ‘+cast(@colid_max as varchar(10))
exec(@sql)

–将其他列的列号加1
set @sql = ‘update syscolumns
set colid = colid + 1
where id = object_id(”’+@tablename+”’)
and colid >= ‘+cast(@colid as varchar(10))
exec(@sql)

–将新列列号复位
set @sql = ‘update syscolumns
set colid = ‘+cast(@colid as varchar(10))+’
where id = object_id(”’+@tablename+”’)
and name = ”’+@colname +””
exec(@sql)
————————————————–
–关闭修改系统表的开关
EXEC sp_configure ‘allow updates’,0 RECONFIGURE WITH OVERRIDE
go 

调用方法：
exec addcolumn ‘表名’,’新列名’,’新列类型’,加到第几个位置
如：
exec addcolumn ‘test’,’id2′,’char(10)’,2
表示将id2这个列加到表test的第二个位置，类型是char(10)。