How would you change a table to InnoDB?

You can change a table to InnoDB using following query.

ALTER TABLE table_name ENGINE innodb;

How would you delete a column?

You can delete a column using following query.

ALTER TABLE table_name DROP column_name;

How would you change a column from VARCHAR(30) to VARCHAR(100)?

ALTER TABLE table_name CHANGE column_name column_name VARCHAR(100).

What is DDL, DML and DCL ?

SQL commands can be divided into three subgroups.

Data Definition Language (DDL) deals with database schema and descriptions of how the data should reside in the database, therefore language statements like CREATE TABLE or ALTER TABLE belong to DDL.

Data Manipulation Language (DML) deals with data manipulation, and therefore includes most common SQL statements such SELECT, INSERT, etc.

Data Control Language (DCL) includes commands such as GRANT, and mostly concerns with rights, permissions and other controls of the database system.