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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) 。