This video explains how to use the ALTER TABLE statement of the Structured Query Language (SQL). It is the second in a series about a subset of SQL known as the Data Definition Language (DDL), which can be used to create and modify the table structures within a relational database. It includes examples of how to use ALTER TABLE to add, drop and alter columns in a table. It also explains how to add a new primary key constraint to an existing column in a table, and how to add a foreign key constrain to an existing column. Some variations between MySQL, SQL Server and Microsoft Access are mentioned.