Dynamic SQL in PL/SQL allows you to construct and execute SQL statements at runtime. This is incredibly useful when you need to build queries based on input parameters or other runtime conditions that aren't known at compile time. The primary mechanism is the EXECUTE IMMEDIATE
statement. This statement takes a string containing the SQL statement as input and executes it directly.
Here's a basic example:
<code class="sql">DECLARE v_sql VARCHAR2(200); v_emp_id NUMBER := 100; v_emp_name VARCHAR2(50); BEGIN v_sql := 'SELECT first_name FROM employees WHERE employee_id = ' || v_emp_id; EXECUTE IMMEDIATE v_sql INTO v_emp_name; DBMS_OUTPUT.PUT_LINE('Employee Name: ' || v_emp_name); END; /</code>
This code snippet dynamically constructs a SELECT
statement based on the value of v_emp_id
. The EXECUTE IMMEDIATE
statement then executes this dynamically generated query, and the result is stored in v_emp_name
. For queries returning multiple rows, you would use a cursor with OPEN FOR
, FETCH
, and CLOSE
statements within a loop. For example:
<code class="sql">DECLARE v_sql VARCHAR2(200); v_dept_id NUMBER := 10; type emp_rec is record (first_name VARCHAR2(50), last_name VARCHAR2(50)); type emp_tab is table of emp_rec index by binary_integer; emp_data emp_tab; i NUMBER; BEGIN v_sql := 'SELECT first_name, last_name FROM employees WHERE department_id = ' || v_dept_id; OPEN emp_cursor FOR v_sql; LOOP FETCH emp_cursor INTO emp_data(i); EXIT WHEN emp_cursor%NOTFOUND; DBMS_OUTPUT.PUT_LINE('Employee Name: ' || emp_data(i).first_name || ' ' || emp_data(i).last_name); i := i 1; END LOOP; CLOSE emp_cursor; END; /</code>
This shows how to handle multiple rows returned by a dynamically generated query. Remember to always handle potential exceptions using EXCEPTION
blocks.
The biggest security risk with dynamic SQL is SQL injection. If user-supplied input is directly concatenated into the SQL statement without proper sanitization, an attacker could inject malicious code, potentially allowing them to read, modify, or delete data they shouldn't have access to.
Mitigation Strategies:
EXECUTE IMMEDIATE
statement supports bind variables using a slightly different syntax:<code class="sql">DECLARE v_emp_id NUMBER := :emp_id; -- Bind variable v_emp_name VARCHAR2(50); BEGIN EXECUTE IMMEDIATE 'SELECT first_name FROM employees WHERE employee_id = :emp_id' INTO v_emp_name USING v_emp_id; -- Binding the value DBMS_OUTPUT.PUT_LINE('Employee Name: ' || v_emp_name); END; /</code>
Performance of dynamic SQL can be impacted by several factors. Here's how to optimize:
EXECUTE IMMEDIATE
with INTO
instead of a cursor. Cursors introduce overhead.Combining the above points, here's a summary of best practices:
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