USE MYSQL Connector for read tag

Other Hardware Home Automation
1

Hi.
I have a problem, I can not read data from the database.
I try to read dta frome database like:

char card;

   sprintf(sqlbuf, "SELECT * FROM rfid.t_main WHERE card = '%s'",card);
   my_conn.cmd_query(sqlbuf);

And i get on Serial Monitor this Error:

Connecting MySQL Server..
Connected to server version 5.5.44-0+deb7u1
Connecting Success!
Error: 156 = You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''' at line 1.
2

please post whole code + links to libraries used.