For the most efficient use of storage, try to use the most precise type in all cases. For example, if an integer column will be used for values in the range from 1 to 99999, MEDIUMINT UNSIGNED is the best type. Of the types that represent all the required values, it uses the least amount of storage.

Accurate representation of monetary values is a common problem. In MySQL, you should use the DECIMAL type. This is stored as a string, so no loss of accuracy should occur. (Calculations on DECIMAL values may still be done using double-precision operations, however.) If accuracy is not too important, the DOUBLE type may also be good enough.

For high precision, you can always convert to a fixed-point type stored in a BIGINT. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary.