I guess you simply run out of memory. Compiling your sketch shows that globals use over 1200bytes of RAM, you then allocate 255 bytes buffers and and a MYSQL cursor also uses at least around 400bytes. The 2kB RAM a UNO have (you didn't specify the type so we expect the default UNO) is filled very fast this way. A simple way to save memory is to use the F() macro in all print() and println() methods with fixed strings. That saves you more than 100 bytes. I'm quite sure the buffers for the SQL commands can be made smaller too. If you have simple tables in the MYSQL DB you can lower the MAXFIELDS constant in the library to save additional memory.