IN CODIO/SQL FORMATE

What is true about SQL constraints?

A constraint is usually associated with a table and they define certain properties that data in a database must comply with.

This is a correct example of a SQL primary key constraint definition: CREATE TABLE table_name (id INT(4), PRIMARY KEY (id));

The following example represents a correct NOT NULL definition: CREATE TABLE table_name (id INT(4) NOT NULL default NULL);

A field with a NULL value is a field with no value.