Database System Concepts, Avi Silberschatz, Henry F. Korth, S. Sudarshan, 2019 (McGraw-Hill) - A fundamental textbook explaining relational database models, schema definition, data types, and integrity constraints.
PostgreSQL 16.2 Documentation: Data Types, Table Creation, and Constraints, The PostgreSQL Global Development Group, 2025 (The PostgreSQL Global Development Group) - Official documentation for PostgreSQL version 16, providing detailed information on defining data types, creating tables, and implementing various integrity constraints like PRIMARY KEY, FOREIGN KEY, and NOT NULL.