Changing a column’s data type also makes use of the ALTER command, this time in conjunction with the MODIFY keyword. So to change the data type of column year from CHAR(4) to SMALLINT (which requires only two bytes of storage and so will save disk space), enter the following:
ALTER TABLE classics MODIFY year SMALLINT;
When you do this, if the conversion of data type makes sense to MySQL, it will automatically change the data while keeping the meaning. In this case, it will change each string to a comparable integer, and so on, as the string is recognizable as referring to an integer.