Converts an internal type into the database-specific SQL type. The defined internal types are: - serial: a number that automatically increments whenever a record is added - smallnum: a small number. needs to be able to hold at least 32,767 possible values (e.g. a 16-bit signed integer) - largenum: a large number. needs to be the same size or larger than a serial type - string: a character field long enough to hold identifiers of objects (e.g. function names) - text: a field that can hold arbitary pieces of text larger than 65536 chars in length.