Use the alter table command to add a column to an existing table.
This is an example adding on a non-null column named author_type, which includes
the constant “primary_author” as the default value, and a null column named
au_publisher to the authors
table.
alter table authors
add author_type varchar(20)
default "primary_author" not null,
au_publisher varchar(40) null
Add Columns Appends Column IDs
alter table adds a column to the table with a column ID that is one greater than the current maximum column ID.
Add NOT NULL Columns
You can add a NOT NULL column to a table. This means that a constant expression, and not a null value, is placed in the column when the column is added. This also ensures that, for all existing rows, the new column is populated with the specified constant expression when the table is created.
Add Constraints
Use alter table to add a constraint to an existing column.