I currently have a few tables in my MySQL database where I declare the id field to be of type SMALLINT(6) (is this correct as SMALLINT only goes to 65535 [length of 5]). I have read here under integer types that when it is UNSIGNED the range increases in the positive to 65535. How will this affect my database if I specifically change the id field's attributes to UNSIGNED? Also, are there problems when going from SMALLINT to MEDIUMINT?
My database type is MyISAM.
You should be fine to switch from SMALLINT to MEDIUMINT - no data will be lost.
A normal integer in MySQL can be any number between a lower limit and an upper limit. With a signed integer, the lower limit is a negative number and the upper is a positive number. A SMALLINT, signed, can be between -32768 and 32767. An unsigned integer, however, is only positive. The lower limit is zero, and the upper limit is 65535.
I usually use INT for IDs, though. I'd rather have an upper limit to an ID field so high that it should never be reached.