You can use any valid C or C++ variable name for a host variable. It is subject to the following restrictions:
Do not use host variable names or external entry names that begin with SQL, RDI, or DSN in any combination of uppercase or lowercase letters. These names are reserved for the database manager. The length of host variable names is limited to 128.
If the name SQL in any combination of uppercase or lowercase letters is used, unpredictable results might occur.