SQLcompatibility

SQL compatibility refers to the degree to which a relational database management system (RDBMS) adheres to the SQL standard and supports interoperable behavior across platforms. It covers the syntax and semantics of data definition and manipulation, data types, functions, constraints, transactions, and the application programming interfaces used to access the database. Standards bodies such as ISO/IEC define successive SQL versions (for example SQL-92, SQL:1999, SQL:2003, SQL:2006, SQL:2011, SQL:2016), with newer releases adding features while preserving backward compatibility. In practice, most engines implement a substantial subset of ANSI SQL and provide vendor-specific extensions to meet performance or domain needs.

Conformance varies by engine. PostgreSQL emphasizes strong standards compliance and extensibility; MySQL aims for broad portability and practicality, though it has historically diverged in areas such as data types and function support; Oracle and SQL Server offer extensive proprietary features via PL/SQL and T-SQL, which can hinder portability. Common divergence points include data types, string and date functions, quoting rules, support for recursive queries and window functions, procedural languages, and transaction isolation behavior. Some engines also expose JSON, XML, or other non-relational features that extend beyond core SQL.

For migration and interoperability, practitioners often rely on ANSI SQL as a baseline, use compatibility modes when available, and prefer portable schemas and queries. Thorough cross-dialect testing, friendly interfaces (ODBC/JDBC), and abstraction layers (such as ORMs) help reduce drift. Ultimately, SQL compatibility is a balance between leveraging a specific engine’s strengths and maintaining portability across systems.