руководство пользователяAs an extension to the SQL-92 standard, MySQL also supports the integer types TINYINT, MEDIUMINT, and BIGINT as listed in the tables above. Another extension is supported by MySQL for optionally specifying the display width of an integer value in parentheses following the base keyword for the type (for example, INT(4)). This optional width specification is used to left-pad the display of values whose width is less than the width specified for the column, but does not constrain the range of values that can be stored in the column, nor the number of digits that will be displayed for values whose width exceeds that specified for the column. When used in conjunction with the optional extension attribute ZEROFILL, the default padding of spaces is replaced with zeros. For example, for a column declared as INT(5) ZEROFILL, a value of 4 is retrieved as 00004. Note that if you store larger values than the display width in an integer column, you may experience problems when MySQL generates temporary tables for some complicated joins, as in these cases MySQL trusts that the data did fit into the original column width.
как говорят в
первоисточнике
надо хранить числа не длинне чем указано в скобках иначе будут проблемы при создании временных таблиц при реализации особо сложных соединений.
