在我的例子中，数据库运行在非标准端口上。检查您正在连接的端口是否与运行数据库的端口相同。如果存在多个SQL server实例，请检查正确的实例。

在我安装的SQL Server 2012开发者版，安装默认设置，我只需要加载SQL Server配置管理器-> SQL Server网络配置-> MSSQLSERVER协议，并将TCP/IP从禁用更改为启用。

在我的例子中，数据库运行在非标准端口上。检查您正在连接的端口是否与运行数据库的端口相同。如果存在多个SQL server实例，请检查正确的实例。

还要检查的一件事是，已命名实例的拼写是否正确!

本文在排除连接问题时非常有用:如何排除连接到SQL Server数据库引擎的故障

I had the same issue with SQL Server 2014 locally installed named instance. Connecting using the FQDN\InstanceName would fail, while connecting using only my hostname\InstanceName worked. For example: connecting using mycomputername\sql2014 worked, but using mycomputername.mydomain.org\sql2014 did not. DNS resolved correctly, TCP/IP was enabled within SQL Configuration Manager, Windows Firewall rules added (and then turned the firewall off for testing to ensure it wasn't blocking anything), but none of those fixed the problem.

最后，我不得不在SQL Server上启动“SQL Server浏览器”服务，这解决了连接问题。

我从未意识到SQL Server浏览器服务实际上是在帮助SQL Server建立连接;我的印象是，当你点击“浏览更多”服务器连接时，它只是帮助填充下拉列表，但它实际上有助于将客户端请求与正确的端口#对齐，如果端口#没有显式分配(类似于网站绑定如何帮助缓解托管多个网站的IIS web服务器上的相同问题)。

这个连接项给了我关于SQLServer浏览器服务的线索:https://connect.microsoft.com/SQLServer/feedback/details/589901/unable-to-connect-on-localhost-using-fqdn-machine-name

when you use wstst05\sqlexpress as a server name, the client code separates the machine name from the instance name and the wstst05 is compared against the netbios name. I see no problem for them to match and the connection is considered local. From there, we retrieve the needed information WITHOUT contacting SQL Browser and connect to the SQL instance via Shared Memory without any problem. when you use wstst05.capatest.local\sqlexpress, the client code fails the comparison of the name (wstst05.capatest.local) to the netbios name (wstst05) and considers the connection "remote". This is by design and we will definitely consider improving this in the future. Anyway, due to considering the connection remote and the fact that it is a named instance, client decides that it needs to use SQLBrowser for name resolution. It attempts to contact SQL Browser on wstst05.capatest.local (UDP port 1434) and apparently that part fails. Hence the error you get.

来自TechNet的“SQL Server Browser”服务的原因(重点是我添加的):https://technet.microsoft.com/en-us/library/ms181087(v=sql.120).aspx

在“使用SQL Server浏览器”部分:

If the SQL Server Browser service is not running, you are still able to connect to SQL Server if you provide the correct port number or named pipe. For instance, you can connect to the default instance of SQL Server with TCP/IP if it is running on port 1433. However, if the SQL Server Browser service is not running, the following connections do not work: Any component that tries to connect to a named instance without fully specifying all the parameters (such as the TCP/IP port or named pipe). Any component that generates or passes server\instance information that could later be used by other components to reconnect. Connecting to a named instance without providing the port number or pipe. DAC to a named instance or the default instance if not using TCP/IP port 1433. The OLAP redirector service. Enumerating servers in SQL Server Management Studio, Enterprise Manager, or Query Analyzer. If you are using SQL Server in a client-server scenario (for example, when your application is accessing SQL Server across a network), if you stop or disable the SQL Server Browser service, you must assign a specific port number to each instance and write your client application code to always use that port number. This approach has the following problems: You must update and maintain client application code to ensure it is connecting to the proper port. The port you choose for each instance may be used by another service or application on the server, causing the instance of SQL Server to be unavailable.

更多信息来自同一篇文章的“SQL Server浏览器如何工作”部分:

Because only one instance of SQL Server can use a port or pipe, different port numbers and pipe names are assigned for named instances, including SQL Server Express. By default, when enabled, both named instances and SQL Server Express are configured to use dynamic ports, that is, an available port is assigned when SQL Server starts. If you want, a specific port can be assigned to an instance of SQL Server. When connecting, clients can specify a specific port; but if the port is dynamically assigned, the port number can change anytime SQL Server is restarted, so the correct port number is unknown to the client. ... When SQL Server clients request SQL Server resources, the client network library sends a UDP message to the server using port 1434. SQL Server Browser responds with the TCP/IP port or named pipe of the requested instance. The network library on the client application then completes the connection by sending a request to the server using the port or named pipe of the desired instance

您所需要做的就是打开服务器防火墙上的相关端口。