MYSQL ALTER TABLE
Changing storage engine; rebuild table; change file_per_table
For example, if t1 is currently not an InnoDB table, this statement changes its storage engine to InnoDB:
If the table is already InnoDB, this will rebuild the table and its indexes and have an effect similar to OPTIMIZE TABLE. You may gain some disk space improvement.
If the value of innodb_file_per_table is currently different than the value in effect when t1 was built, this will convert to (or from) file_per_table.
ALTER COLUMN OF TABLE
Change auto-increment value
Changing an auto-increment value is useful when you don't want a gap in an AUTO_INCREMENT column after a massive deletion.
For example, you got a lot of unwanted (advertisement) rows posted in your table, you deleted them, and you want to fix the gap in auto-increment values. Assume the MAX value of AUTO_INCREMENT column is 100 now. You can use the following to fix the auto-increment value.