Constraints that add information about how a column can be used are invoked after specifying the data type for a column. They can be used to tell the database to reject inserted data that does not adhere to a certain restriction.

The statement below sets constraints on the students table:

CREATE TABLE students (
   id INTEGER PRIMARY KEY,
   name TEXT UNIQUE,
   date_of_birth TEXT NOT NULL,
   date_of_death TEXT DEFAULT 'Not Applicable'
);

• PRIMARY KEY columns can be used to uniquely identify the row. Attempts to insert a row with an identical value to a row already in the table will result in a constraint violation which will not allow you to insert the new row.

• UNIQUE columns have a different value for every row. This is similar to PRIMARY KEY except a table can have many different UNIQUE columns.

• NOT NULL columns must have a value. Attempts to insert a row without a value for a NOT NULL column will result in a constraint violation and the new row will not be inserted.

• DEFAULT columns take an additional argument that will be the assumed value for an inserted row if the new row does not specify a value for that column.