Dynamic SQL in Oracle | Complete Overview

In the context of Dynamic SQL in Oracle, there are several common terms and concepts that developers should be familiar with. These terms help in understanding how dynamic SQL works and how it can be effectively utilized in an Oracle environment.

Common Terms in Dynamic SQL

1. Dynamic SQL:

Definition: SQL queries that are constructed and executed at runtime, rather than being hard-coded in the application. Dynamic SQL allows flexibility in query formation based on runtime conditions like user input, table names, column names, or schema changes.

Example: Constructing a SQL query based on user input where table and column names are dynamic.

1. Static SQL:

Definition: A traditional SQL statement that is predefined and hard-coded in the application. Unlike dynamic SQL, static SQL is fixed and doesn't change based on runtime conditions.

Example: SELECT * FROM employees WHERE department_id = 10;

1. EXECUTE IMMEDIATE:

Definition: A PL/SQL command used to execute dynamic SQL statements. It allows you to execute a SQL query that is constructed at runtime.

Syntax:

EXECUTE IMMEDIATE 'SQL statement';

Example:

EXECUTE IMMEDIATE 'CREATE TABLE my_table (id INT)';

1. Bind Variables:

Definition: Placeholders used in dynamic SQL statements that are replaced with actual values at runtime. Bind variables help avoid SQL injection attacks and optimize query performance by allowing Oracle to reuse execution plans.

Example:

EXECUTE IMMEDIATE 'SELECT * FROM employees WHERE department_id = :dept_id' USING dept_id_value;

Here, :dept_id is a bind variable.

1. Parsing:

Definition: The process of analyzing and converting the dynamic SQL query into an execution plan by the database engine. Parsing happens at runtime for dynamic SQL, which involves syntactical analysis and checks.

Parsing can be expensive in terms of performance if done repeatedly, which is why using bind variables can improve efficiency.

1. SQL Injection:

Definition: A type of attack that allows attackers to execute arbitrary SQL code by manipulating user input. This is a risk when using dynamic SQL if user input is directly embedded into the SQL statement.

Mitigation: Always use bind variables to avoid SQL injection attacks.

1. DBMS_SQL Package:

Definition: A built-in Oracle package that provides an advanced way to work with dynamic SQL. It allows for more flexibility, such as working with cursors, binding variables dynamically, and executing SQL statements with complex structures.

Example:

DECLARE
v_cursor INTEGER;
v_sql VARCHAR2(1000);
BEGIN
v_sql := 'SELECT * FROM employees WHERE department_id = :dept_id';
v_cursor := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(v_cursor, v_sql, DBMS_SQL.NATIVE);
DBMS_SQL.BIND_VARIABLE(v_cursor, ':dept_id', 10);
DBMS_SQL.EXECUTE(v_cursor);
DBMS_SQL.CLOSE_CURSOR(v_cursor);
END;

1. Cursor:

Definition: A database object used to retrieve, store, and process query results. In dynamic SQL, cursors are used to manage the result set of queries executed at runtime.

Explicit Cursors are used to work with dynamic SQL and can be manipulated programmatically in PL/SQL.

1. OPEN CURSOR:

Definition: A command used in DBMS_SQL to allocate a cursor that can be used to parse and execute dynamic SQL statements.

Example: v_cursor := DBMS_SQL.OPEN_CURSOR;

1. BIND VARIABLES:

Definition: Dynamic SQL uses bind variables to provide runtime values for placeholders in the SQL query. Bind variables are important for both security (to prevent SQL injection) and performance (as they can reuse the SQL execution plan).

Example:

EXECUTE IMMEDIATE 'SELECT * FROM employees WHERE department_id = :dept_id' USING 10;

1. SQL String Concatenation:

Definition: The process of combining multiple strings into a single SQL query. While this can be used to build dynamic SQL, it should be done cautiously to avoid SQL injection vulnerabilities.

Example:

DECLARE
v_sql VARCHAR2(1000);
BEGIN
v_sql := 'SELECT * FROM ' || table_name || ' WHERE id = :id';
EXECUTE IMMEDIATE v_sql USING bind_id;
END;

1. NATIVE Parsing:

Definition: Refers to the parsing method used by Oracle's DBMS_SQL package to process the SQL statement. It means that the SQL statement will be parsed in its original form (native SQL).

Example: DBMS_SQL.PARSE(cursor_id, sql_statement, DBMS_SQL.NATIVE);

1. Dynamic Query Generation:

Definition: The process of creating an SQL query at runtime, based on dynamic inputs or conditions. For example, you might need to build a query where the columns or tables are not known in advance.

Example:

DECLARE
v_sql VARCHAR2(1000);
BEGIN
v_sql := 'SELECT * FROM ' || table_name || ' WHERE department = :dept';
EXECUTE IMMEDIATE v_sql USING 'IT';
END;

1. PL/SQL Block:

Definition: A PL/SQL block (which consists of declarations, execution, and exception handling sections) can be used to encapsulate dynamic SQL execution, especially when the SQL statement needs to be generated or modified dynamically during execution.

1. Dynamic DDL:

Definition: Dynamic DDL refers to creating or altering database objects such as tables, indexes, or views dynamically at runtime. EXECUTE IMMEDIATE is commonly used to execute DDL statements dynamically.

Example:

EXECUTE IMMEDIATE 'CREATE TABLE new_table (id INT, name VARCHAR2(100))';

1. SELECT INTO with Dynamic SQL:

Definition: When using dynamic SQL to fetch a single value into a variable, you can use the SELECT INTO clause in conjunction with EXECUTE IMMEDIATE.

Example:

DECLARE
v_count INT;
BEGIN
EXECUTE IMMEDIATE 'SELECT COUNT(*) FROM employees' INTO v_count;
END;

Conclusion:

Dynamic SQL in Oracle provides great flexibility for executing SQL queries at runtime based on dynamic conditions. However, it requires careful handling of SQL injection risks and performance considerations. Understanding key concepts like bind variables, parsing, cursors, and the DBMS_SQL package will help you write efficient and secure dynamic SQL in Oracle.