问题：用SQL语句修改表字段为主键 怎么写(不能删除,再添加,因为已经有数据)

　　Peak Wong：

　　删除主键和数据没关系. 删除后再创建.

　　SQL code

以下是引用片段：
　　如何取主键字段名称及字段类型
　　--得到主键字段名
　　1:
SELECT TABLE_NAME,COLUMN_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE TABLE_NAME<>'dtproperties'
　　2:
EXEC sp_pkeys @table_name='表名'
　　3:
select o.name as 表名,c.name as 字段名,k.colid as 字段序号,k.keyno as 索引顺序,t.name as 类型
from sysindexes i
join sysindexkeys k on i.id = k.id and i.indid = k.indid
join sysobjects o on i.id = o.id
join syscolumns c on i.id=c.id and k.colid = c.colid
join systypes t on c.xusertype=t.xusertype
where o.xtype = 'U' and o.name='要查询的表名'
and exists(select 1 from sysobjects where xtype = 'PK' and parent_obj=i.id and name = i.name)
order by o.name,k.colid

　　需要注意：先确认表中现存数据在该字段上唯一。