Which construct defines higher-level rules for data items, including nullability and keys?

• A. Data Dictionary
• B. Constraint
• C. Attribute
• D. Block

Answer: (B)

Constraints are the rules that govern how data items can be stored and related. They define higher-level requirements for values, such as whether a column can be null (NOT NULL), which values must be unique (UNIQUE), which field serves as a primary key, how rows relate to rows in other tables (FOREIGN KEY), and other conditions (CHECK). These rules are what enforce data integrity across the database, guiding what is allowed for data items and their relationships. A data dictionary, by contrast, stores metadata about data items—names, types, lengths, and descriptions—rather than enforcing the rules themselves. An attribute is a data item or column, not a mechanism for expressing these governance rules, and a block isn’t a standard term for this concept.