Review the following pages in this tutorial: CONSTRAINTS, PRIMARY KEY, and FOREIGN KEY.
Constraints are used in DDL commands to incorporate rules for a table. Constraints are defined with Create or Alter Table commands. The NOT NULL constraint defines a rule that a column cannot store a null value. Null and NOT NULL are also used in the Select command to find NULL or NOT NULL values in fields or columns. This rule specifies that the value in a field or column of a table is different for each record or row. Primary Key is a constraint that defines a field or column that uniquely identifies each row (is NOT NULL and is Unique). A foreign key is a column in a table that contains a value that is a primary key of another table. Thus, a foreign key links a referenced table to a referencing table. Default specifies a default value for a column. This constraint checks that a value in a column satisfies a specified condition.