如何在C#中的一个连接字符串中连接两个数据库？

Question

通常,当我需要使用C#连接到数据库时,我将使用以下命令例程:
- 定义一个mysql连接.
- 打开一个mysql连接.
- 定义一个sql语句/查询.
- 使用MySqlCommand执行查询.

示例代码:

string con1 = "server=<db1 IP>;User Id=user;password=password;Persist Security Info=True;database=db1";
string con2 = "server=<db2 IP>;User Id=user;password=password;Persist Security Info=True;database=db2";
MySqlConnection cn1 = new MySqlConnection(con1);
MySqlConnection cn2 = new MySqlConnection(con2);
MySqlCommand com

cn1.Open();
string sql = "some query";
com = new MySqlCommand(sql, cn1);
com.executeNonQuery();  
cn1.Close();

我上面的问题是在我使用MySqlCommand命令的部分,因为它是指示数据库连接的位置,以便它现在将查询哪个数据库

MySqlCommand com = new MySqlCommand(sql, con);  

其中sql是sql语句,con是用于查询的连接.

如何在一个sql语句中查询两个数据库？
请考虑以下事项:(我正在使用MySQL)

- I have two databases, db1 and db2.
- db1 is located in City A
- db1 is located in City B
- Both databases have one table (tbl) and they both have the same structure.
- Table structure for tbl:
    +-------------+--------------+------+-----+---------+-------+
    | Field       | Type         | Null | Key | Default | Extra |
    +-------------+--------------+------+-----+---------+-------+
    | id          | int(9)       | NO   | PRI |         |       |
    | ref_no      | int(9)       | NO   |     |         |       |
    | name        | varchar(10)  | YES  |     | NULL    |       |
    +-------------+--------------+------+-----+---------+-------+
- I want to run a query on db1.tbl against db2.tbl
- Example query: "select ref_no from db1.tbl where ref_no not in (select ref_no from db2.tbl)"  

或者还有另一种解决这类问题的方法吗？......

Answer 1

string con = "server=localhost;user=root;pwd=1234;";

using (MySqlConnection cn1 = new MySqlConnection(con))
{
    MySqlCommand cmd = new MySqlCommand();
    cmd.Connection = cn1;
    cn1.Open();

    cmd.CommandText = sql;
    MySqlDataAdapter da = new MySqlDataAdapter();
    ....
}

sql语句:

select a.ref_no from db1.tbl a where a.ref_no not in (select b.ref_no from db2.tbl b)

您可以一次查询多个数据库.

更新

我认为唯一的选择是同时创建2个连接,并通过C#在2服务器之间传递数据.