Dynamic SQL

Enhanced Definition

Dynamic SQL in the z/OS context, primarily within DB2 for z/OS, refers to SQL statements that are constructed and prepared for execution at application runtime rather than being pre-compiled and bound during program development. This allows for greater flexibility, as the full text of the SQL statement, including table names, column names, or predicates, can be determined or modified based on program logic or user input during execution. Dynamic SQL refers to SQL statements that are constructed, prepared, and executed at runtime by an application program or interactive tool, rather than being pre-compiled and bound during application development. In the context of DB2 for z/OS, this allows for greater flexibility, as the exact SQL statement can vary based on user input or application logic.

Key Characteristics

- Runtime Preparation: SQL statements are parsed, validated, and an access path is determined by DB2 at the time of execution, not during a separate bind step.
- Flexibility: Allows applications to construct and execute SQL statements whose exact form (e.g., WHERE clause, ORDER BY clause, table names) is not known until the program runs.
- Host Language Integration: Requires specific host language constructs (e.g., PREPARE, EXECUTE IMMEDIATE, OPEN with USING DESCRIPTOR, FETCH) in languages like COBOL, PL/I, or Java (JDBC/ODBC).
- Performance Overhead: Each execution of a PREPARE statement incurs CPU and I/O overhead as DB2 must parse the statement, check authorization, and determine an optimal access path.
- Security Implications: Without proper handling (e.g., using parameter markers), dynamic SQL is highly susceptible to SQL injection attacks, where malicious input alters the intended query.
- No Static Bind Package: Unlike static SQL, dynamic SQL does not rely on a pre-existing DB2 package created during a bind step; the access path is determined dynamically.

Use Cases

- Ad-hoc Query Tools: Applications that allow users to build and execute arbitrary SQL queries against DB2 databases, such as reporting tools or data exploration utilities.
- Generic Data Access Layers: Frameworks or middleware that provide a generalized interface to database operations, where the specific SQL varies based on application logic or configuration.
- Dynamic Reporting: Generating reports where the selection criteria, columns displayed, or sorting order are determined by user input at runtime.
- Application Servers: Many application servers (e.g., WebSphere Application Server) use dynamic SQL via JDBC/ODBC drivers to interact with DB2 for z/OS based on application logic.
- Database Administration Utilities: Tools that need to execute schema changes or data manipulation commands based on administrative input.

Related Concepts

Dynamic SQL is often contrasted with Static SQL, where SQL statements are fully defined and embedded in the application code, then pre-compiled and bound into a DB2 package before execution. While static SQL offers predictable performance due to pre-determined access paths, dynamic SQL provides flexibility at the cost of potential runtime overhead. Both are fundamental methods for applications to interact with DB2 for z/OS, which is the primary relational database management system on the mainframe. The DB2 Optimizer plays a crucial role in dynamic SQL by determining the most efficient access path for each statement at runtime.

Best Practices:

Use Parameter Markers: Always use parameter markers (?) in dynamic SQL statements for variable data to prevent SQL injection and allow DB2 to reuse prepared statements more effectively.
Cache Prepared Statements: For frequently executed dynamic SQL, PREPARE the statement once