The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.
The ALTER TABLE statement is also used to add and drop various constraints on an existing table.
SYNTAX
ALTER TABLE table_name
ADD column_name datatype;
EXAMPLE
ALTER TABLE Customers
ADD Email varchar(255);
ALTER TABLE Employees ADD Email varchar(255);
To delete a column in a table, use the following syntax (notice that some database systems don’t allow deleting a column):
ALTER TABLE Customers
DROP COLUMN Email;