Which of the following relational database constructs is used to ensure valid values are entered...

 A constraint is a rule or a restriction that is applied to a column or a table in a relational database to ensure that only valid values are entered. Constraints help to maintain the integrity, accuracy, and consistency of the data. For example, a constraint can be used to specify that a column must not contain null values, or that a column must contain unique values, or that a column must match a value in another table12. References := CompTIA IT Fundamentals (ITF+) Study Guide, 2nd Edition, Chapter 5: Database Fundamentals3; Constraints in Relational Database Model - Online Tutorials Library