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# FAQ

#### 1. When encoutered with the error "failed to connect to server", what can I do?

The client may encounter connection errors. Please follow the steps below for troubleshooting:

1. On the server side, execute `systemctl status taosd` to check the status of taosd service. If taosd is not running, start it and retry connecting.
2. Make sure you have used the correct server IP address to connect to.
3. Ping the server. If no response is received, check your network connection.
4. Check the firewall setting, make sure the TCP/UDP ports from 6030-6039 are enabled.
5. For JDBC, ODBC, Python, Go connections on Linux, make sure the native library libtaos.so are located at /usr/local/lib/taos, and /usr/local/lib/taos is in the LD_LIBRARY_PATH.
6. For JDBC, ODBC, Python, Go connections on Windows, make sure driver/c/taos.dll is in the system search path (or you can copy taos.dll into C:\Windows\System32)
7. If the above steps can not help, try the network diagnostic tool nc to check if TCP/UDP port works check UDP port：`nc -vuz {hostIP} {port}` check TCP port on server: `nc -l {port}` check TCP port on client: `nc {hostIP} {port}`

#### 2. Why I get "Invalid SQL" error when a query is syntactically correct?

If you are sure your query has correct syntax, please check the length of the SQL string, it shall be less than 64KB.

#### 3. Why I could not delete a super table?

Please make sure there are no tables under the super table. You could not delete a super table which still has associated tables.

#### 4. Does TDengine support validation queries?

For the time being, TDengine does not have a specific set of validation queries. However, TDengine comes with a system monitoring database named 'sys', which can usually be used as a validation query object.

#### 5. Can I delete or update a record that has been written into TDengine?

The answer is NO. The design of TDengine is based on the assumption that records are generated by the connected devices, you won't be allowed to change it. But TDengine provides a retention policy, the data records will be removed once their lifetime is passed.

#### 6. How do I create a table with more than 250 columns?

For a single table, the maximum number of columns is 250. If for some reason, 250 columns are still not quite enough, our suggestion is to split the huge table into several smaller ones.

#### 7. What is the most efficient way to write data to TDengine?

TDengine supports several different writing regimes. The most efficient way to write data to TDengine is to use batch inserting. For details on batch insertion syntax, please refer to Taos SQL

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