Ошибка подключения к базе sql

Abstract

This topic was updated and moved into SQL Server Books Online in June. 2016. See
https://msdn.microsoft.com/library/mt750266.aspx 

Original topic follows:

This is an exhaustive list of troubleshooting techniques to use when you cannot connect to the SQL Server Database Engine. These steps are not in the order of the most likely problems which you probably already tried. These steps
are in order of the most basic problems to more complex problems. These steps assume that you are connecting to SQL Server from another computer by using the TCP/IP protocol, which is the most common situation. These steps are written for SQL Server 2008 R2
with a client running Windows 7, however the steps generally apply to other versions of SQL Server and other operating systems with only slight modifications.

These instructions are particularly useful when troubleshooting the «Connect to Server» error, which can be Error Number: 11001 (or 53), Severity: 20, State: 0

• «A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections.
  »
• «(provider: Named Pipes Provider, error: 40 — Could not open a connection to SQL Server) (Microsoft SQL Server, Error: 53)» or «(provider: TCP Provider, error: 0 — No such host is known.) (Microsoft SQL Server, Error: 11001)»

This error usually means that the SQL Server computer can’t be found or that the TCP port number is either not known, or is not the correct port number, or is blocked by a firewall.

Not included

• This topic does not include information about SSPI errors. For SSPI errors, see How to troubleshoot the «Cannot generate SSPI context» error message

• This topic does not include information about Kerberos errors. For help, see Microsoft Kerberos Configuration Manager for SQL Server.

• This topic does not include information about SQL Azure Connectivity.

Gathering Information about the Instance of SQL Server

1. Confirm the instance of the SQL Server Database Engine is installed and running.

   1. Logon to the computer hosting the instance of SQL Server.
   2. On the Start menu, point to All Programs, point to Microsoft SQL Server 2008 R2, point to Configuration Tools, and then click SQL Server Configuration Manager.
   3. Using Configuration Manager, in the left pane select SQL Server Services. In the right-pane confirm that the instance of the Database Engine is present and running. The instance named MSSQLSERVER is
      a default (unnamed) instance. There can only be one default instance. Other (named) instances will have their names listed between the parentheses. SQL Server Express uses the name SQLEXPRESS as the instance name unless someone named it something
      else during setup. Make a note of the name of the instance that you are trying to connect to. Also, confirm that the instance is running, by looking for the green arrow. If the instance has a red square, right-click the instance and then click Start.
      It should turn green.        
   4. If you are attempting to connect to a named instance, make sure the SQL Server Browser service is running.
2. Get the IP Address of the computer.
   1. On the Start menu, click Run. In the Run window type cmd, and then click OK.
   2. In the command prompt window, type ipconfig and then press enter. Make a note of the IPv4 Address and the IPv6 Address. (SQL Server can connect using the older IP version 4 protocol
      or the newer IP version 6 protocol. Your network could allow either or both. Most people start by troubleshooting theIPv4 address. It’s shorter and easier to type.)
3. Get the TCP port number used by SQL Server. In most cases you are connecting to the Database Engine from another computer using the TCP protocol.
   1. Using SQL Server Management Studio on the computer running SQL Server, connect to the instance of SQL Server. In Object Explorer, expand Management, expand SQL Server Logs, and then double-click
      the current log.
   2. In the Log Viewer, click the Filter button on the toolbar. In the Message contains text box, type server is listening on, click Apply filter, and then click OK.
   3. A message similar to Server is listening on [ ‘any’ <ipv4> 1433] should be listed. This message indicates that this instance of SQL Server is listening on all the computers IP Addresses (for IP version 4) and
      is listening to TCP port 1433. (TCP port 1433 is usually the port used by the Database Engine. Only one instance of SQL Server can use a port, so if there is more than one instance of SQL Server installed, some instances must use other port numbers.)

Enable Protocols

In many installations of SQL Server, connecting to the Database Engine from another computer is not enabled unless an administrator uses Configuration Manager to enable it. To enable connections from another computer:

1. On the Start menu, point to All Programs, point to Microsoft SQL Server 2008 R2, point to Configuration Tools, and then click SQL Server Configuration Manager.
2. Using Configuration Manager, in the left pane expand SQL Server Network Configuration (or SQL Server Network Configuration (32bit)), and then select the instance of SQL Server that you want to
   connect to. The right-pane lists the connection protocols available. Shared Memory is normally enabled. It can only be used from the same computer, so most installations leave Shared Memory enabled. To connect to SQL Server from another computer you will normally
   use TCP/IP. If TCP/IP is not enabled, right-click TCP/IP, and then click Enable.
3. If you changed the enabled setting for any protocol you must restart the Database Engine. In the left pane select SQL Server Services. In the right-pane, right-click the instance of the Database Engine, and then
   click Restart.

Testing TCP/IP Connectivity

Connecting to SQL Server by using TCP/IP requires that Windows can establish the connection.

1. On the Start menu, click Run. In the Run window type cmd, and then click OK.
2. In the command prompt window, type ping and then the IP Address of the computer that is running SQL Server. For example, ping 192.168.1.101 using an IPv4 address, or ping fe80::d51d:5ab5:6f09:8f48%11 using
   an IPv6 address. (You must replace the numbers after ping with the IP addresses on your computer.)
3. If your network is properly configured you will receive a response such as Reply from <IP address>. If you receive an error such as «Destination host unreachable.» or «Request timed out.»
   then TCP/IP is not correctly configured. (Check that the IP address was correct and was correctly typed.) Errors at this point could indicate a problem with the client computer, the server computer, or something about the network such as a router.  For more
   information, see How to Troubleshoot Basic TCP/IP Problems.
4. Next, if the ping test succeeded using the IP address, a test that the computer name can be resolved to the TCP/IP address. On the client computer, in the command prompt window, type ping and then the computer
   name of the computer that is running SQL Server. For example, ping newofficepc.
5. If you receive an error such as «Destination host unreachable.» or «Request timed out.» you might have old (stale) name resolution information cached on the client computer. Type ipconfig
   /flushdns to clear the DNS (Dynamic Name Resolution) cache. Then ping the computer by name again. With the DNS cache empty, the client computer will check for the newest information about the IP address for the server computer.
6. If your network is properly configured you will receive a response such as Reply from <IP address>. If you can successfully ping the server computer by IP address but receive an error such as «Destination
   host unreachable.» or «Request timed out.» when pinging by computer name, then name resolution is not correctly configured. (For more information, see How to Troubleshoot Basic TCP/IP
   Problems.) Successful name resolution is not required to connect to SQL Server, but if the computer name cannot be resolved, then connections must be made specifying the IP address. This is not ideal, but name resolution can be fixed later.

Testing a Local Connection

Before troubleshooting a connection problem from another computer, first test your ability to connect from a client application on the computer that is running SQL Server. This procedure
uses SQL Server Management Studio. Management Studio might not have been installed when you installed the Database Engine. You can install Management Studio from the SQL Server CD by running setup and selecting the Management Tools option. If you are running
SQL Server Express, you can download the free SQL Server Management Studio Express from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=08e52ac2-1d62-45f6-9a4a-4b76a8564a2b.
(If Management Studio is not available you can test the connection using the sqlcmd.exe utility which is installed with the Database Engine.)

1. Logon to the computer where SQL Server is installed, using a login that has permission to access SQL Server. (SQL Server 2008 installation requires at least one login to be specified as a SQL Server Administrator. If you do not
   know an administrator, see Troubleshooting: Connecting to SQL Server When System Administrators Are Locked Out.)

2.  On the Start menu, point to All Programs, point to Microsoft SQL Server 2008 R2, and then click SQL Server Management Studio.

3. In the Connect to Server dialog box, in the Server type box, select Database Engine. In the Authentication box, select Windows Authentication.
   In the Server name box, type one of the following:

   Connecting to:	Type:	Example:
   Default instance	The computer name	ACCNT27
   Named Instance	The computer nameinstance name	ACCNT27PAYROLL

   Note: When connecting to a SQL Server from a client application on the same computer, the shared memory protocol is used. Shared memory is a type of local named pipe, so sometimes errors regarding pipes are encountered.

   If you receive an error at this point, you will have to resolve it before proceeding. There are many possible things that could be a problem. Your login might not be authorized to connect. Your default database might be missing.

   Note: Some error messages passed to the client intentionally do not give enough information to troubleshoot the problem. This is a security feature to avoid providing an attacker with information about SQL Server.
   To view the complete information about the error, look in the SQL Server error log. The details are provided there. If you are receiving error 18456 «Login failed for user», Books Online topic http://msdn.microsoft.com/en-us/library/cc645917.aspx contains
   additional information about error codes. And Aaron Bertrand’s blog has a very extensive list of error codes athttp://www2.sqlblog.com/blogs/aaron_bertrand/archive/2011/01/14/sql-server-v-next-denali-additional-states-for-error-18456.aspx. 

4.  If you can connect using shared memory, test connecting using TCP. You can force a TCP connection by specifying tcp: before the name. For example:

   Connecting to:	Type:	Example:
   Default instance	tcp: The computer name	tcp:ACCNT27
   Named Instance	tcp: The computer name/instance name	tcp:ACCNT27PAYROLL

   If you can connect with shared memory but not TCP, then you must fix the TCP problem. The most likely issue is that TCP is not enabled. To enable TCP, See the Enable Protocols steps above.

5.  If your goal is to connect with an account other than an administrator account, once you can connect as an administrator, try the connection again using the Windows Authentication login or the SQL Server Authentication login that
   the client application will be using.

Opening a Port in the Firewall

Beginning with Windows XP Service Pack 2, the Windows firewall is turned on and will block connections from another computer. To connect using TCP/IP from another computer, on the SQL Server computer you must configure the firewall
to allow connections to the TCP port used by the Database Engine. If you are connecting to a named instance or a port other than TCP port 1433, you must also open the UDP port 1434 for the SQL Server Browser service. For step by step instruction on opening
a port in the Windows firewall, see How to: Configure a Windows Firewall for Database Engine Access.

Testing the Connection

Once you can connect using TCP on the same computer, it’s time to try connecting from the client computer. You could theoretically use any client application, but to avoid additional complexity, install the SQL Server Management
tools on the client and make the attempt using SQL Server Management Studio.

1. On the client computer, using SQL Server Management Studio, attempt to connect using the IP Address and the TCP port number in the format IP address comma port number. For example, 192.168.1.101,1433 If this
doesn’t work, then you probably have one of the following problems:

• Ping of the IP address doesn’t work, indicating a general TCP configuration problem. Go back to the section Testing TCP/IP Connectivity.
• SQL Server is not listening on the TCP protocol. Go back to the section Enable Protocols.
• SQL Server is listening on a port other than the port you specified. Go back to the section Gathering Information about the Instance of SQL Server.
• The SQL Server TCP port is being blocked by the firewall. Go back to the section Opening a Port in the Firewall.

2. Once you can connect using the IP address and port number, attempt to connect using the IP address without a port number. For a default instance, just use the IP address. For a named instance, use the IP address and the instance
name in the format IP address backslash instance name, for example 192.168.1.101PAYROLL If this doesn’t work, then you probably have one of the following problems:

• If you are connecting to the default instance, it might be listening on a port other than TCP port 1433, and the client isn’t attempting to connect to the correct port number.  
• If you are connecting to a named instance, the port number is not being returned to the client. 

Both of these problems are related to the SQL Server Browser service, which provides the port number to the client. The solutions are:

• Start the SQL Server Browser service. Go back to the section Gathering Information about the Instance of SQL Server, section 1.b.
• The SQL Server Browser service is being blocked by the firewall. Open UDP port 1434 in the firewall. Go back to the section Opening a Port in the Firewall.
• The UDP port 1434 information is being blocked by a router. UDP communication (datagrams) are not designed to pass through routers. This keeps the network from getting filled with low priority traffic. You might be able to configure your router to forward
  UDP traffic, or you can decide to always provide the port number when you connect.
• If the client computer is using Window 7 or Windows Server 2008, (or a more recent operating system,) the UDP traffic might be dropped by the client operating system because the response from the server is returned from a different IP address than was queried.
  This is a security feature blocking «loose source mapping.» For more information, see the Multiple Server IP Addresses section of the Books Online topic Troubleshooting: Timeout
  Expired. You might be able to configure the client to use the correct IP address, or you can decide to always provide the port number when you connect.

3. Once you can connect using the IP address  (or IP address and instance name), attempt to connect using the computer name (or computer name and instance name). Put tcp: in front of the computer name to force
a TCP/IP connection. For example, for a default instance use something like tcp:ACCNT27 For a named instance usesomething like tcp:ACCNT27PAYROLL If you could connect using the IP address but not using the computer name,
then you have a name resolution problem. Go back to the section Testing TCP/IP Connectivity, section 4.

4. Once you can connect using the computer name forcing TCP, attempt connecting using the computer name but not forcing TCP. For example, for a default instance use just the computer name such as ACCNT27 For a
named instance use the computer name and instance name like ACCNT27PAYROLL If you could connect using whileforcing TCP, but not without forcing TCP, then the client is probably using another protocol (such as named pipes).

a. On the client computer, using SQL Server Configuration Manager, in the left-pane expand SQL Native Client 10.0 Configuration, and then select Client Protocols.

b. On the right-pane, Make sure TCP/IP is enabled. If TCP/IP is disabled, right-click TCP/IP and then click Enable.

c. Make sure that the protocol order for TCP/IP is a smaller number that the named pipes or VIA protocols. Generally you should leave Shared Memory as order 1 and TCP/IP as order 2. Shared memory is only used when the client and
SQL Server are running on the same computer. All enabled protocols are tried in order until one succeeds, except that shared memory is skipped when the connection is not to the same computer. 

See Also

Another important place to find an extensive amount of SQL Server General & Database Engine related articles is the TechNet Wiki itself. The best entry point is SQL
Server General & Database Engine Resources on the TechNet Wiki

Обработка исключений

Подключение к базе данных представляет собой одно из слабых мест в работе программы. В силу самых разных причин клиент может не получить доступ к базе данных. Поэтому при создании приложения следует обязательно включать обработку исключений и возможность предоставления пользователю информации о них.

Для получения специализированных сообщений при возникновении ошибок подключения к базе данных Microsoft SQL Server используются классы SqlException и SqlErro r. Объекты этих классов можно применять для перехвата номеров ошибок, возвращаемых базой данных (таблица 4.2):

Дополнительно вводятся уровни ошибок SQL Server, позволяющие охарактеризовать причину проблемы и ее сложность (таблица 4.3):

Создайте новое Windows-приложение и назовите его «ExceptionsSQL». Свойству Size формы устанавливаем значение «600;380». Добавляем на форму элемент управления DataGrid, его свойству Dock устанавливаем значение «Fill». Перетаскиваем элемент Panel, определяем следующие его свойства:

На панели размещаем четыре текстовых поля, надпись и кнопку:

Интерфейс приложения готов. Подключаем пространство имен для работы с базой данных:

using System.Data.SqlClient;

Объекты ADO .NET и весь блок обработки исключений помещаем в обработчик кнопки «Соединение»:

private void btnConnect_Click(object sender, System.EventArgs e)
{
	SqlConnection conn = new SqlConnection();
	label1.Text = "";
	try
	{
//conn.ConnectionString = "workstation id=9E0D682EA8AE448;data source="(local)
//";" + "persist security info=True;initial catalog=Northwind;
//user id=sa;password=12345";

//Строка ConnectionString в качестве параметров 
//будет передавать значения, введенные в текстовые поля:
		conn.ConnectionString = 
		"initial catalog=" + txtInitialCatalog.Text + ";" +
		"user id=" + txtUserID.Text + ";" +
		"password=" + txtPassword.Text + ";" +
		"data source=" + txtDataSource.Text + ";" +
		"workstation id=9E0D682EA8AE448;persist security info=True;";
		SqlDataAdapter dataAdapter = new SqlDataAdapter("SELECT * FROM 
		 Customers", conn);
		DataSet ds = new DataSet();
		conn.Open();
		dataAdapter.Fill(ds);
		dataGrid1.DataSource = ds.Tables[0].DefaultView;
	}
	catch (SqlException OshibkiSQL)
	{
		foreach (SqlError oshibka in OshibkiSQL.Errors)
		{
			//Свойство Number объекта oshibka возвращает 
			//номер ошибки SQL Server
			switch (oshibka.Number)
			{
			case 17:
			label1.Text += "nНеверное имя сервера!";
			break;
			case 4060:
			label1.Text += "nНеверное имя базы данных!";
			break;
			case 18456:
			label1.Text += "nНеверное имя пользователя или пароль!";
			break;
			}
			//Свойство Class объекта oshibka возвращает 
			//уровень ошибки SQL Server,
			//а свойство Message - уведомляющее сообщение
			label1.Text +="n"+oshibka.Message + "
			 Уровень ошибки SQL Server: " + oshibka.Class;			}
	}
	//Отлавливаем прочие возможные ошибки:
	catch (Exception ex)
	{
		label1.Text += "nОшибка подключения: " + ex.Message;
	}
	finally
	{
		conn.Dispose();
	}
}

Закомментированная строка подключения содержит обычное перечисление параметров. При отладке приложения будет легче сначала добиться наличия подключения, а затем осуществлять привязку параметров, вводимых в текстовые поля. Запускаем приложение. При вводе неверных параметров в надпись выводятся соответствующие сообщения, а при правильных параметрах элемент DataGrid отображает данные (рис. 4.11):

В программном обеспечении к курсу вы найдете приложение Exceptions SQL (CodeGlava2 ExceptionsSQL).

Скопируйте папку приложения ExceptionsSQL и назовите ее «ExceptionsMDB». Удаляем с панели на форме имеющиеся текстовые поля и добавляем три новых:

Изменяем пространство имен для работы с базой данных:

Обработчик кнопки «Соединение» будет выглядеть так:

private void btnConnect_Click(object sender, System.EventArgs e)
{
	OleDbConnection conn = new OleDbConnection();
	label1.Text = "";
	try
	{
// conn.ConnectionString = @"Provider=""Microsoft.Jet.OLEDB.4.0"";
//Data Source=""D:UchebnikCodeGlava2BDwithUsersP.mdb"";
Jet OLEDB:System database=""D:UchebnikCodeGlava2BDWorkFile.mdw"";
User ID=Adonetuser;Password=12345;Jet OLEDB:Database Password=98765;";
				
//Строка ConnectionString в качестве параметров 
//будет передавать значения, введенные в текстовые поля:
		conn.ConnectionString = 
		"Jet OLEDB:Database Password=" + txtDataBasePassword.Text 
		 + ";" + "User ID=" + txtUserID.Text + ";" +
		 "password=" + txtPassword.Text + ";" +

				@"Provider=""Microsoft.Jet.OLEDB.4.0"";Data 		
	Source=""D:UchebnikCodeGlava2BDwithUsersP.mdb"";
Jet OLEDB:System database=""D:UchebnikCodeGlava
BDWorkFile.mdw"";";

		OleDbDataAdapter dataAdapter = 
		 new OleDbDataAdapter("SELECT * FROM Туристы", conn);
		DataSet ds = new DataSet();
		conn.Open();
		dataAdapter.Fill(ds);
		dataGrid1.DataSource = ds.Tables[0].DefaultView;
	}
	catch (OleDbException oshibka)
	{
		//Пробегаем по всем ошибкам
		for (int i=0; i < oshibka.Errors.Count; i++)
		{
			label1.Text+= "Номер ошибки " + i 
			 + "n" + "Сообщение: " + 
			 oshibka.Errors[i].Message + "n" +
			 "Номер ошибки NativeError: " + 
			 oshibka.Errors[i].NativeError + "n" +
			 "Источник: " + oshibka.Errors[i].Source + 
			 "n" + "Номер SQLState: " + 
			 oshibka.Errors[i].SQLState + "n";
			}
		}
		//Отлавливаем прочие возможные ошибки:
		catch (Exception ex)
		{
			label1.Text += "nОшибка подключения: " + 
			 ex.Message;
		}
		finally
		{
			conn.Dispose();
		}
}

Запускаем приложение (рис. 4.12). Свойство Message возвращает причину ошибки на русском языке, поскольку установлена русская версия Microsoft Office 2003. Свойство NativeError (внутренняя ошибка) возвращает номер исключения, генерируемый самим источником данных. Вместе или по отдельности со свойством SQL State их можно использовать для создания переключателя, предоставляющего пользователю расширенную информацию (мы это делали в приложении ExceptionsSQL) .

Кроме обработки исключений, здесь следует обратить внимание на интерфейс, предоставляющий возможность пользователю вводить сведения своей учетной записи. Теперь в целях безопасности приложения не нужно включать пароль в код при использовании мастеров, поскольку строка соединения будет генерироваться динамически. Разумеется, ваше приложение может иметь другой интерфейс — например, окно соединения будет появляться в дочерней форме.

В программном обеспечении к курсу вы найдете приложение Exceptions MDB (CodeGlava2 ExceptionsMDB).

Вопрос:

Возникают проблемы при подключении к SQL Server, при этом в сообщении об ошибке встречается фрагмент:

1. «Ошибка подключения к серверу

   SQL

   «;

2. «Ошибка подключения к экземпляру сервера

   SQL

   «;

3. «При установлении соединения с

   SQL

   Server произошла ошибка, связанная с сетью или определенным экземпляром. Сервер не найден или недоступен»;

4. «Error 26»;

5. «Error: 40».

Ответ:

Для устранения проблемы попробуйте воспользоваться приведенной ниже инструкцией.

Инструкция по устранению наиболее часто возникающих проблем с подключением к SQL Server:

1) Проверьте, что имя сервера задано верно (в частности, если компьютер был переименован):

Также проверьте, что имя экземпляра (в данном случае – SQLEXPRESS) задано верно. При необходимости допишите его вручную в поле «Сервер баз данных».
Примечание. Если SQL Server был установлен автоматически при установке Business Studio, то имя экземпляра должно быть SQLEXPRESS.

2) Если сервер SQL находится не на локальном компьютере, то проверьте, что компьютер включен и подключен к сети.

3) Проверьте, что SQL Server запущен и настроен верно. Далее приведен пример для SQL Server 2014 (подходит и для других версий SQL Server, начиная с 2008)¹⁾:

a. Откройте Пуск->Все программы->Microsoft SQL Server 2014->Диспетчер конфигурации SQL Server 2014.

b. Проверьте, что запущена служба SQL Server (на скриншоте пример для экземпляра SQLEXPRESS), т.е. что она находится в состоянии «Работает»:

c. Проверьте, что запущена служба обозревателя SQL Server (SQL Server Browser), т.е. что она находится в состоянии «Работает»:

d. Проверьте, что удаленные соединения разрешены, т.е. что состояние протоколов «Именованные каналы» и «TCP/IP» для рассматриваемого экземпляра SQL Server – «Включено»:

4) Проверьте, что никакие брандмауэры на компьютере с сервером SQL не блокируют соединение со службой SQL. То же касается и антивирусов, т.к. некоторые из них имеют функции брандмауэров. Обратитесь к документации по этим средствам. Для проверки можно временно отключить их.
Далее приведен пример для встроенного брандмауэра Windows 7:

a. Откройте Пуск->Панель управления->Брандмауэр Windows, перейдите на закладку «Разрешить запуск программы или компонента через брандмауэр Windows» (для Windows 10 необходимо вызвать контекстное меню правой кнопкой мыши от кнопки «Пуск»->Сетевые подключения->Брандмауэр Windows->Брандмауэр и безопасность сети->Разрешить работу с приложением через брандмауэр) и нажмите кнопку «Изменить параметры».

b. Добавьте следующие программы, как показано на рисунке (повторив поз. 1-3 для каждой из программ)²⁾:

• C:Program Files (x86)Microsoft

  SQL

  ServerMSSQL12.SQLEXPRESSMSSQLBinnsqlservr.exe (служба SQL Server)

• C:Program Files (x86)Microsoft

  SQL

  Server90Sharedsqlbrowser.exe (cлужба обозревателя SQL Server)

c. Перезапустите вышеуказанные службы.

5) Возможно, имеются проблемы с разрешением сетевых имен. Попробуйте в окне Свойств подключения к базе данных в поле «Сервер базы данных» указать вместо имени компьютера его IP-адрес. Например, так:

В случае локального сервера SQL укажите IP-адрес 127.0.0.1, в случае удаленного – узнайте у администратора сети.

When I experienced this error in Visual Studio,

“A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 — Could not open a connection to SQL Server)”

…it was during the execution of the following C# code, which was attempting to obtain my SQL Server data to display it in a grid. The break occurred exactly on the line that says connect.Open():

        using (var connect = Connections.mySqlConnection)
        {
            const string query = "SELECT Name, Birthdate, Narrative FROM Friends";
            using (var command = new SqlCommand(query, connect))
            {
                connect.Open();
                using (var dr = command.ExecuteReader())
                {
                    while (dr.Read())
                    {
                        // blah
                    }
                }
            }
        }

It was inexplicable because the SQL query was very simple, I had the right connection string, and the database server was available. I decided to run the actual SQL query manually myself in SQL Management Studio and it ran just fine and yielded several records. But one thing stood out in the query results: there was some improperly encoded HTML text inside a varchar(max) type field within the Friends table (specifically, some encoded comment symbols of the sort <!-- lodged within the «Narrative» column’s data). The suspect data row looked like this:

Name    Birthdate    Narrative
====    =========    ============== 
Fred    21-Oct-79    <!--HTML Comment -->Once upon a time...

Notice the encoded HTML symbol «<«, which stood for a «<» character. Somehow that made its way into the database and my C# code could not pick it up! It failed everytime right at the connect.Open() line! After I manually edited that one row of data in the database table Friends and put in the decoded «<» character instead, everything worked! Here’s what that row should have looked like:

Name    Birthdate    Narrative
====    =========    ============== 
Fred    21-Oct-79    <!--HTML Comment -->Once upon a time...

I edited the one bad row I had by using this simple UPDATE statement below. But if you had several offending rows of encoded HTML, you might need a more elaborate UPDATE statement that uses the REPLACE function:

UPDATE Friends SET Narrative = '<!--HTML Comment -->Once upon a time...' WHERE Narrative LIKE '&lt%'

So, the moral of the story is (at least in my case), sanitize your HTML content before storing it in the database and you won’t get this cryptic SQL Server error in the first place! (Uh, properly sanitizing/decoding your HTML content is the subject of another discussion worthy of a separate StackOverflow search if you need more information!)