• PRQL a simple, powerful, pipelined SQL replacement
• Интерактивный учебник по SQL

Wikipedia

• Check constraint is a type of integrity constraint in SQL which specifies a requirement that must be met by each row in a database table.

Books

• SQL Query Design Patterns and Best Practices. A practical guide to writing readable and maintainable SQL queries using its design patterns by Steve Hughes, Dennis Neer, Dr. Ram Babu Singh, Shabbir H. Mala, Leslie Andrews, Chi Zhang - 270 pages