getting fatal errors when work with lots of data - mysql

Question

Below i have attach my code. in my code it stops on the line :
cmdUpdate.executenonquery();
and after few seconds exception is generated and the catch block is executed.in catch block it says
fatal error
I dont know why this error occurs.
And one main important thing is firstly i have executed this code without any LIMIT condition in the query specified in cmdPayment command.then it does not generate any exception.
But when i remove LIMIT condition from query then it generates exception.
This query will select data from table which has around 20.
int freeMonths=0;
string subscriptionTxnId = null;
string paymentType=null;
string subscriptionDate = null;
string userid = null;
string date = "2014-02-01 00:00:00";
using (MySqlConnection connPayment = new MySqlConnection(connString))
{
connPayment.Open();
freeMonths = 0;
MySqlCommand cmdPayment = new MySqlCommand("SELECT userid,subscr_txn_id,DATE_FORMAT(subscr_date,'%Y-%m-%d %k:%i:%s'),payment_type from payment limit 10000",connPayment);
MySqlDataReader readerPayment;
readerPayment = cmdPayment.ExecuteReader();
while (readerPayment.Read())
{
try
{
freeMonths = 0;
if (readerPayment.GetValue(0) != DBNull.Value && readerPayment.GetValue(1) != DBNull.Value && readerPayment.GetValue(2) != DBNull.Value && readerPayment.GetValue(3) != DBNull.Value)
{
userid = readerPayment.GetString(0);
subscriptionTxnId = readerPayment.GetString(1);
subscriptionDate = readerPayment.GetString(2);
paymentType = readerPayment.GetString(3);
using (MySqlConnection connPaymentDetails = new MySqlConnection(connString))
{
connPaymentDetails.Open();
MySqlCommand cmdPaymentDetails = new MySqlCommand("select free_months from payment_details where userid = '" + userid + "' AND subscr_txn_id ='" + subscriptionTxnId + "' AND payment_date = '" + subscriptionDate + "' ", connPaymentDetails);
MySqlDataReader readerPaymentDetails = cmdPaymentDetails.ExecuteReader();
if (readerPaymentDetails.HasRows)
{
readerPaymentDetails.Read();
freeMonths = readerPaymentDetails.GetInt32(0);
}
}
string query = null;
MySqlConnection conUpdate = new MySqlConnection(connString);
if (paymentType == "annual" || paymentType == "Annual")
{
//MySqlCommand cmd001 = new MySqlCommand("")
query = "update payment set end_date = (select DATE_ADD(DATE_ADD('" + subscriptionDate + "' ,INTERVAL 1 YEAR),INTERVAL '" + freeMonths + "' MONTH)) where subscr_date = '" + subscriptionDate + "' AND userid = '" + userid + "' AND subscr_txn_id ='" + subscriptionTxnId + "' ";
}
else if (paymentType == "quarter" || paymentType == "Quarter" || paymentType == "Quarterly")
{
freeMonths += 3;
query = "update payment set end_date = (select DATE_ADD('" + subscriptionDate + "', INTERVAL '" + freeMonths + "' MONTH)) where subscr_date = '" + subscriptionDate + "' AND userid = '" + userid + "' AND subscr_txn_id ='" + subscriptionTxnId + "' ";
}
else if (paymentType == "month" || paymentType == "Month" || paymentType == "Monthly")
{
freeMonths += 1;
query = "update payment set end_date = (select DATE_ADD('" + subscriptionDate + "', INTERVAL '" + freeMonths + "' MONTH)) where subscr_date = '" + subscriptionDate + "' AND userid = '" + userid + "' AND subscr_txn_id ='" + subscriptionTxnId + "' ";
}
MySqlCommand cmdUpdate = new MySqlCommand(query, conUpdate);
conUpdate.Open();
cmdUpdate.ExecuteNonQuery();
int num = 0;
if (num == 0)
{
}
}
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
}
}