Error : fatal error encountered during command execution

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C#

MySQL

I want to insert data in a database table from two other tables

Whenever I run my code, it would return this particular error

Quote:

An unhandled exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll - Additional information: Fatal error encountered during command execution.

exception image:
https://i.stack.imgur.com/I2Px3.png[^]

which is pointed at

cmd.ExecuteNonQuery();

What could be the problem here? Here's my code

What I have tried:

private void Merge_Tables_Click(object sender, EventArgs e)
            {

                string MergeTables = (@"INSERT INTO exercices (annee,AA,region) 
                SELECT e1.annee,e1.AA,e2.region
                FROM bilan e1 JOIN data e2
                ON e1.siren = e2.siren");

                string serverIp = "127.0.0.1";
                string username = "root";
                string password = "root";
                string databaseName = "altares";

                string dbConnectionString = string.Format("server={0};uid={1};pwd={2};database={3};SslMode=none", serverIp, username, password, databaseName);

                var conn = new MySqlConnection(dbConnectionString);
                conn.Open();

                var cmd = new MySqlCommand(MergeTables, conn);

                cmd.ExecuteNonQuery();

                MessageBox.Show("Table exercices now with data !!");

            }