MySQL database tables with SMALLINT id fields

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开发者_运维百科 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.

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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.

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The main problem with using UNSIGNED types is that if you ever do any arithmetic that results in a negative number you end up with a large number instead. However, for ID's this is unlikely to be a problem for and I often use them to reduce memory usage a bit.

I can't think of any problems with using a SMALLINT then changing to MEDIUMINT because widening a data-type doesn't usually cause issues.