Executing Dynamic Query Statements

This section describes how to execute dynamic query statements. GaussDB provides two methods for dynamic query: EXECUTE IMMEDIATE and OPEN FOR. The former dynamically executes SELECT statements while the latter combines the use of cursors. If you need to store query results in a dataset, use OPEN FOR.

EXECUTE IMMEDIATE

Figure 1 shows the syntax diagram.

Figure 1 EXECUTE IMMEDIATE dynamic_select_clause::=

Figure 2 shows the syntax diagram for using_clause.

Figure 2 using_clause::=

The syntax is described as follows:

define_variable: specifies a variable to store query results.
USING IN bind_argument: specifies the variable whose value is passed to the dynamic SQL statement. The variable is used when a dynamic placeholder exists in dynamic_select_string.
USING OUT bind_argument: specifies the variable that stores a value returned by the dynamic SQL statement.
- In query statements, INTO and OUT cannot coexist.
- A placeholder name starts with a colon (:) followed by digits, characters, or strings, corresponding to bind_argument in the USING clause.
- bind_argument can only be a value, variable, or expression. It cannot be a database object such as a table name, column name, and data type. That is, bind_argument cannot be used to transfer schema objects for dynamic SQL statements. If a stored procedure needs to transfer database objects through bind_argument to construct dynamic SQL statements (generally, DDL statements), you are advised to use double vertical bars (||) to concatenate dynamic_select_clause with a database object.
- A dynamic PL/SQL block allows duplicate placeholders. That is, a placeholder can correspond to only one bind_argument in the USING clause. When the GUC parameter behavior_compat_options is set to dynamic_sql_compat, the bind arguments in the USING clause are matched in sequence based on the placeholder sequence. Duplicate placeholders will not be identified as the same placeholder.
- The IMMEDIATE keyword is used only for syntax compatibility and has no actual meaning.

Example:

    
     
       
       
         gaussdb=# DROP SCHEMA IF EXISTS hr CASCADE;
gaussdb=# CREATE SCHEMA hr;
gaussdb=# SET CURRENT_SCHEMA = hr;
gaussdb=# CREATE TABLE staffs 
(
  staff_id NUMBER, 
  first_name VARCHAR2,
  salary NUMBER
);
gaussdb=# INSERT INTO staffs VALUES (200, 'mike', 5800);
gaussdb=# INSERT INTO staffs VALUES (201, 'lily', 3000);
gaussdb=# INSERT INTO staffs VALUES (202, 'john', 4400);

-- Retrieve values from dynamic statements (INTO clause).
gaussdb=# DECLARE
   staff_count  VARCHAR2(20);
BEGIN
   EXECUTE IMMEDIATE 'select count(*) from hr.staffs'
      INTO staff_count;
   dbe_output.print_line(staff_count);
END;
/
3
ANONYMOUS BLOCK EXECUTE
-- Pass and retrieve values (the INTO clause is used before the USING clause).
gaussdb=# CREATE OR REPLACE PROCEDURE dynamic_proc
AS
   staff_id     NUMBER(6) := 200;
   first_name   VARCHAR2(20);
   salary       NUMBER(8,2);
BEGIN
   EXECUTE IMMEDIATE 'select first_name, salary from hr.staffs where staff_id = :1'
       INTO first_name, salary
       USING IN staff_id;
   dbe_output.print_line(first_name || ' ' || salary);
END;
/
CREATE PROCEDURE
-- Call the stored procedure.
gaussdb=# CALL dynamic_proc();
mike 5800.00
 dynamic_proc 
--------------

(1 row)

-- Delete the stored procedure.
gaussdb=# DROP PROCEDURE dynamic_proc;

        

      

    
   

OPEN FOR

Dynamic query statements can be executed by using OPEN FOR to open dynamic cursors.

Figure 3 shows the syntax diagram.

Figure 3 open_for::=

Parameter description:

cursor_name: specifies the name of the cursor to be opened.
dynamic_string: specifies the dynamic query statement.
USING value: applies when a placeholder exists in dynamic_string.

For details about use of cursors, see Cursors.

Example:

     
      
        
        
          gaussdb=# CREATE SCHEMA hr;
gaussdb=# SET CURRENT_SCHEMA = hr;
gaussdb=# CREATE TABLE staffs 
(
  section_id NUMBER,
  first_name VARCHAR2,
  phone_number VARCHAR2, 
  salary NUMBER
  );
gaussdb=# INSERT INTO staffs VALUES (30, 'mike', '13567829252', 5800);
gaussdb=# INSERT INTO staffs VALUES (40, 'john', '17896354637', 4000);

gaussdb=# DECLARE
    name          VARCHAR2(20);
    phone_number  VARCHAR2(20);
    salary        NUMBER(8,2);
    sqlstr        VARCHAR2(1024);

    TYPE app_ref_cur_type IS REF CURSOR; -- Define the cursor type.
    my_cur app_ref_cur_type; -- Define the cursor variable.
    
BEGIN
    sqlstr := 'select first_name,phone_number,salary from hr.staffs
         where section_id = :1';
    OPEN my_cur FOR sqlstr USING '30'; -- Open the cursor. USING is optional.
    FETCH my_cur INTO name, phone_number, salary; -- Retrieve the data.
    WHILE my_cur%FOUND LOOP
          dbe_output.print_line(name||'#'||phone_number||'#'||salary);
          FETCH my_cur INTO name, phone_number, salary;
    END LOOP;
    CLOSE my_cur; -- Close the cursor.
END;
/
mike#13567829252#5800.00
mike#13567829252#5800.00
ANONYMOUS BLOCK EXECUTE

         

       

     
    