When you create a table in MySQL, you must specify the data type for each column. This appendix documents all the column types that MySQL provides as of version 5.5.22 (current this writing).
In this reference, many column types can accept optional
parameters to further customize how data for the column is
stored or displayed. First, there are the
D parameters, which are indicated (in square brackets
when optional) immediately following the column type name.
M is used to specify the display
size (that is, maximum number of characters) to be used by values in the
column. In most cases, this will limit the range of values that may be
specified in the column.
M may be any integer between 1 and 255. ...