(mysql.info.gz) Choosing types

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 11.6 Choosing the Right Type for a Column
 =========================================
 
 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.
 

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(mysql.info.gz) Storage requirements

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