Constraints

Constraints that add information about how a column can be used are invoked after specifying the data type for a column.

The statement below sets constraints on the celebs table.

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

1. 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.

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

3. 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.

4. 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.