How to Resolve SQL Variable Name Conflicts in Loops?

Variable Name Conflict in SQL

When executing a SQL statement, you may encounter an error message stating that a variable name has already been declared and must be unique within a query batch or stored procedure. This can occur in situations where multiple parameters with the same name are added in a loop.

For instance, consider the following code snippet:

for (long counter = from; counter <= to; counter++) {
  string upd = "update card set LockState=@lockstate, card_descr=@card_descr where [cardNumber] = N'{0}'";
  rwd.command.CommandText = upd;
  rwd.command.Parameters.Add(new SqlParameter("@LockState", SqlDbType.NVarChar)).Value = 1;
  rwd.command.Parameters.Add(new SqlParameter("@card_descr", SqlDbType.NVarChar)).Value = txt_desc2.Text;
  rwd.connection.Open();
  rwd.command.ExecuteScalar();
  rwd.connection.Close();
}

In this code, the variable @LockState is added to the SQL command's parameters multiple times within the loop. This violates the rule that variable names must be unique.

To resolve this issue, you can either add the parameters outside the loop and then update their values within the loop, or use the Parameters.Clear() method after each loop iteration to remove the previously added parameters.

Here's an example of adding the parameters outside the loop:

rwd.command.Parameters.Add(new SqlParameter("@LockState", SqlDbType.NVarChar));
rwd.command.Parameters.Add(new SqlParameter("@card_descr", SqlDbType.NVarChar));

for (long counter = from; counter <= to; counter++) {
  string upd = "update card set LockState=@lockstate, card_descr=@card_descr where [cardNumber] = N'{0}'";
  rwd.command.CommandText = upd;

  rwd.command.Parameters["@LockState"].Value = 1;
  rwd.command.Parameters["@card_descr"].Value = txt_desc2.Text;

  rwd.connection.Open();
  rwd.command.ExecuteScalar();
  rwd.connection.Close();
}

Alternatively, you can use Parameters.Clear() within the loop:

for (long counter = from; counter <= to; counter++) {
  rwd.command.Parameters.Clear();

  string upd = "update card set LockState=@lockstate, card_descr=@card_descr where [cardNumber] = N'{0}'";
  rwd.command.CommandText = upd;
  rwd.command.Parameters.Add(new SqlParameter("@LockState", SqlDbType.NVarChar)).Value = 1;
  rwd.command.Parameters.Add(new SqlParameter("@card_descr", SqlDbType.NVarChar)).Value = txt_desc2.Text;

  rwd.connection.Open();
  rwd.command.ExecuteScalar();
  rwd.connection.Close();
}

By ensuring unique variable names within your SQL parameters, you can avoid conflicting variable declarations and successfully execute your statement.

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