Question

I am using C#(asp.net). I have two tables(data and details) in a same database.

Table “data”

id | chap | unit |
  ----------------
  1| chap1|unit1 |
  2| chap2|unit2 |
  3| chap3|unit3 |

Table “details”

id| code| num |
----------------
 1|abc  |2    |
 2|efg  |3    |
 3|hij  |1    |

Now I want to fetch a value from “num” where code=”efg” (in table “details”). And use the same value (3) to fetch data from table “data” by id. I am using this code.

OleDbConnection conn = new OleDbConnection(*** ...... *****); 
       OleDbCommand cmd;
       OleDbDataReader reader;
        String query = String.Format("select num from details where code="efg");
        cmd = new OleDbCommand(query, conn);
        reader = cmd.ExecuteReader();
        int num = int.Parse(reader.GetValue(0).ToString());

        query = String.Format("select chap from data where id={0}",num);
         cmd = new OleDbCommand("select lesson from data where id=3", conn);
        reader = cmd.ExecuteReader();


        Label1.Text = reader.GetValue(0).ToString();

But it shows error. It shows “No data exists for the row/column.”

Answer 1

You can use

SELECT d.chap, d.unit
FROM data d INNER JOIN details de
    ON d.id = de.num
WHERE de.code = 'efg'

or

SELECT d.chap, d.unit
FROM data d INNER JOIN details de
    ON d.id = de.num
   AND de.code = 'efg'

More: if you’re using SQL-Server, use SqlConnection in place of OleDbConnection.
More: don’t format your query joining strings, numbers, dates and so on; use SqlParameter so you don’t have to worry about types and formatting!!