Publisher Summary This chapter discusses constraints and their use to implement “referential integrity.” Constraints perform validation on data both within and between tables and, to a lesser degree, with views. Primary and foreign key constraints are probably the most significant of constraints. These constraints can be used to enforce the validity of relationships between rows in tables. A primary key reference is placed on a unique identifying column in a superset entity. Referential Integrity can be enforced using constraints or triggers, or can be coded at the application level. Six types of constraints are presented. Some of these are always inline constraints, whereas others can be either inline or out-of-line, depending on how they are defined. A constraint can be applied to an individual column in a table or, in some cases, to a table as a whole. Constraints can even be used in views. A check constraint applies a condition to an expression of a row such as checking a column value. Constraint syntax for the CREATE VIEW command is analyzed. The way in which constraints are handled for the CREATE VIEW command is the same as for the CREATE TABLE command.