Question

I am trying to develop an application using ADO.NET. I have two tables in two different databases. Each of them have a portion of the complete data. Now I need to write a query such that I am able to fetch the complete record from the two tables. As an example say table 1 has Index_no,emp_ID and contact no. Table 2 has index_no, emp_name, salary and dept. The index_no is same for the same record portion in each table.
Following is the code to fetch all records where salary <20000.

sqlCmd2 = new SqlCommand("SELECT * FROM table1 WHERE Index_No =@Index_No",
                         TestCon);
int i = 0; 
while (reader.Read()) {
    ListBox1.Items.Add(reader.GetInt32(0) + "  -   " + 
      reader.GetString(1) + "  -   " + reader.GetString(2));
    //  Name.Text += reader["Name"] + "<br />"; ;
    // Depart.Text += reader["Depart"] + "<br />"; ;
    array1[i] = reader.GetInt32(0);
    i++;
}

sqlCmd2.Parameters.Add("@Index_No", System.Data.SqlDbType.Decimal);
i = 0;
do {
    sqlCmd2.Parameters["@Index_No"].Value = array1[i];
    reader1 = sqlCmd2.ExecuteReader();
    reader1.Read();
    ListBox2.Items.Add(reader1.GetString(1));
    i++;
} while (i < array1.Length);

The problem with this is that I am getting only one record info from table2 while for table 1 I get all the desired record info. The do-while loop seems to terminate after one iteration only.

Answer 1

It depends, what two different databases means. If the two databases run on the same SQL-Server, it is easy to access the table of the other database

 SELECT * FROM OtherDb.dbo.TableOnOtherDb

If not, I would suggest you to create a database link between the two databases (see SO question How do I create and query linked database servers in SQL Server?). Then you can access the other table as above.

In both cases you can use a JOIN query to join the two tables. Then you will need only one data reader

SELECT
    A.Index_no, A.emp_ID, A.contact_no,
    B.emp_name, B.salary, B.dept
FROM
    table1 A
    INNER JOIN table2 B
        ON A.Index_no = B.Index_no
WHERE
    B.salary < 20000
ORDER BY
    B.emp_name

---

UPDATE

If the number of involved records is small, then querying the second table only once by using a IN clause would be far more efficient, if you cannot link the two servers for some reason.

SELECT *
FROM table1
WHERE Index_no IN (4,12,17,30,112,167)

You would create this SQL statement like this

string[] stringArray = array1
    .Select(i => i.ToString())
    .ToArray();
string list = String.Join(",", stringArray);
string sql = "SELECT * FROM table1 WHERE Index_No IN (" + list + ")";

The idiomatic way of doing the second loop would be (not do while)

for (int i = 0; i < array1.Length; i++) {
    ...
}