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Types of Cursors Available in Oracle PL/SQL

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Types of Cursors Available in Oracle PL/SQL

Here's a breakdown of the various pointers to SQL result sets you can implement in your code.

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Let's take a look at the different ways you can define and use cursors (pointers to SQL result sets) in PL/SQL, including: implicit cursor, explicit cursor, cursor expressions, cursor variables, DBMS_SQL cursor handles. 

Check out the LiveSQL script that demonstrates the points made in this post.

SELECT-INTO (AKA, Implicit Cursor)

The SELECT-INTO statement is called an implicit cursor because we write the SELECT statement, and the PL/SQL engine takes care of all the cursor operations for us implicitly: open, fetch, close.

SELECT-INTO is the best (smallest amount of code, best performance) way to fetch a single row.

Here's an example, based on the standard HR schema.

PACKAGE BODY employee_mgr
IS
   FUNCTION onerow (employee_id_in IN employees.employee_id%TYPE)
      RETURN hr.employees%ROWTYPE
   IS
      onerow_rec   hr.employees%ROWTYPE;
   BEGIN
      SELECT *
        INTO onerow_rec
        FROM employees
       WHERE employee_id = employee_id_in;

      RETURN onerow_rec;
   END;
END;

SELECT-INTO famously can raise (at least) two exceptions:

  1. NO_DATA_FOUND: zero rows were found for the specified WHERE clause.
  2. TOO_MANY_ROWS: two or more rows were found for the specified WHERE clause.

When you write a SELECT-INTO, you should decide how you want to handle these exceptions — and you should also be aware of how different they are. In many situations, NO_DATA_FOUND doesn't even indicate a real error. It probably just reflects a data state: no row yet in database for that primary or unique key. TOO_MANY_ROWS almost always indicates a serious data error, such a disabled constraint or missing unique index, leading to bad data.

So usually when I write functions that fetch single rows (or individual column values from a row), I write something more like this:

PACKAGE BODY employee_mgr
IS
   FUNCTION onerow (employee_id_in IN employees.employee_id%TYPE)
      RETURN hr.employees%ROWTYPE
   IS
      onerow_rec   hr.employees%ROWTYPE;
   BEGIN
      SELECT *
        INTO onerow_rec
        FROM employees
       WHERE employee_id = employee_id_in;

      RETURN onerow_rec;
   EXCEPTION
      WHEN NO_DATA_FOUND
      THEN
         RETURN NULL;
      WHEN TOO_MANY_ROWS
      THEN
         /* Replace with your logging routine, such as logger.log_error */
         log_error ('employee_mgr.onerow', 'Too many rows for '|| employee_id_in);
   END;
END;

SELECT-INTO Documentation

CURSOR xxx IS (AKA, Explicit Cursor)

With an explicit cursor, you associate a SELECT statement with a cursor that you declare in a declaration section or package specification. You can even define a parameter list for your cursor, just as you would a function. Then you get to control all aspects or cursor operation: open, fetch, close. If that's what you want. Generally, do not use explicit cursors for single row lookups; implicits are simpler and faster.

But because you have total control, you also have lots of flexibility. Take a look at some examples. First, I create and populate a table:

CREATE TABLE endangered_species 
( 
   common_name    VARCHAR2 (100), 
   species_name   VARCHAR2 (100) 
)
/

BEGIN
   INSERT INTO endangered_species
        VALUES ('Amur Leopard', 'Panthera pardus orientalis');

   INSERT INTO endangered_species
        VALUES ('Hawksbill Turtle', 'Eretmochelys imbricata');

   INSERT INTO endangered_species
        VALUES ('Javan Rhino', 'Rhinoceros sondaicus');

   COMMIT;
END;


I can then define and use an explicit cursor as follows:

DECLARE 
   CURSOR species_cur 
   IS 
        SELECT * 
          FROM endangered_species 
      ORDER BY common_name; 

   l_species   species_cur%ROWTYPE; 
BEGIN 
   OPEN species_cur; 
   FETCH species_cur INTO l_species; 
   CLOSE species_cur; 
END;


Notice that I define my record based on the cursor, not the underlying table. That way, if the SELECT list ever changes in the cursor's query, I don't have to change the record declaration. It's anchored back to the cursor.

Then I open, fetch, and close. That block shows the basic operations, but a reminder: If all you are doing is fetching a single row, once, like this, use SELECT-INTO.

There is, however, more that I can do. Here's an example of a cursor with a parameter:

DECLARE 
   CURSOR species_cur (filter_in IN VARCHAR2) 
   IS 
        SELECT * 
          FROM endangered_species 
         WHERE species_name LIKE filter_in 
      ORDER BY common_name; 

   l_species   species_cur%ROWTYPE; 
BEGIN 
   OPEN species_cur ('%u%'); 
   FETCH species_cur INTO l_species; 
   CLOSE species_cur; 

   /* Use same cursor a second time, avoiding copy-paste of SQL */ 
   OPEN species_cur ('%e%'); 
   FETCH species_cur INTO l_species; 
   CLOSE species_cur; 

   /* I can even use it in a cursor FOR loop */ 
   FOR rec IN species_cur ('%u%') 
   LOOP 
      DBMS_OUTPUT.PUT_LINE (rec.common_name); 
   END LOOP; 
END;


So you can see that I can reuse my explicit cursor, but I cannot reuse an implicit cursor in the same way. With an implicit, I would have to put the SELECT-INTO inside a function and call the function. But, hey, that's what I recommend you do anyway!

Another nice feature of the explicit cursor is that you can declare the cursor in the specification, but not show the SQL. Then you define the cursor (including the SELECT) in the package body — as shown in next step. This way, you hide the details of the query and "force" the user of the cursor to simply rely on its documented specification.

CREATE PACKAGE species_pkg 
IS 
   CURSOR species_cur 
      RETURN endangered_species%ROWTYPE; 
END;
/

CREATE PACKAGE BODY species_pkg 
IS 
   CURSOR species_cur 
      RETURN endangered_species%ROWTYPE 
   IS 
        SELECT * 
          FROM endangered_species 
      ORDER BY common_name; 
END;
/

Explicit Cursor Documentation

Cursor FOR Loop

The cursor FOR Loop is one of my favorite PL/SQL features. No need to open, fetch, close. Just tell the PL/SQL engine you want to work with each row returned by the query. Plus, with your optimization level set to 2 (the default) or higher, this code is automatically optimized to return 100 rows with each fetch (similar to BULK COLLECT).

You can embed the SELECT statement within the loop header, or you can declare an explicit cursor and reference it by name in the loop header. That means you can, again, parameterize the cursor and reuse it in multiple loops.

BEGIN 
   FOR rec IN (  SELECT * 
                   FROM endangered_species 
               ORDER BY common_name) 
   LOOP 
      DBMS_OUTPUT.put_line (rec.common_name); 
   END LOOP; 
END;
/

DECLARE 
   CURSOR species_cur 
   IS 
        SELECT * 
          FROM endangered_species 
      ORDER BY common_name; 

   PROCEDURE start_conservation_effort 
   IS 
   BEGIN 
      DBMS_OUTPUT.put_line ('Remove human presence'); 
   END; 
BEGIN 
   FOR rec IN species_cur 
   LOOP 
      DBMS_OUTPUT.put_line (rec.common_name); 
   END LOOP; 

   FOR rec IN species_cur 
   LOOP 
      start_conservation_effort; 
   END LOOP; 
END;
/


Things to keep in mind with cursor FOR loops:

  • Never use a cursor FOR loop to fetch a single row. It's a lazy way to avoid declaring the INTO variable or record, and that's bad lazy.
  • If you need to iterate through rows of data but then conditionally exit the loop under certain data conditions, use a WHILE or simple loop with an explicit cursor. Why? Because...
  • Any kind of FOR loop is saying, implicitly, "I am going to execute the loop body for all iterations defined by the loop header." (N through M or SELECT) Conditional exits mean the loop could terminate in multiple ways, resulting in code that is hard to read and maintain.

Cursor FOR Loop Documentation

Cursor Variables

A cursor variable is, well, just that: a variable pointing back to a cursor/result set. Some really nice aspects of cursor variables are that you can associate a query with a cursor variable at runtime (useful with both static and dynamic SQL) and that you can pass the cursor variable as a parameter or function RETURN value (specifically: you can pass a cursor variable back to a host language like Java for consumption).

Here's a package and script that demonstrates several cursor variable features:


CREATE OR REPLACE PACKAGE refcursor_pkg 
IS 
   /* Use this "strong" REF CURSOR to declare cursor variables whose 
      queries return data from the endangered_species table. */ 

   TYPE endangered_species_t IS REF CURSOR 
      RETURN endangered_species%ROWTYPE; 

   /* Use a "weak" REF CURSOR to declare cursor variables whose 
      queries return any number of columns. 

      Or use the pre-defined SYS_REFCURSOR, see example below. 
   */ 

   TYPE weak_t IS REF CURSOR; 

   FUNCTION filtered_species_cv (filter_in IN VARCHAR2) 
      RETURN endangered_species_t; 

   /* Return data from whatever query is passed as an argument. */ 
   FUNCTION data_from_any_query_cv (query_in IN VARCHAR2) 
      RETURN weak_t; 

   /* Return data from whatever query is passed as an argument. 
      But this time, use the predefined weak type. */ 
   FUNCTION data_from_any_query_cv2 (query_in IN VARCHAR2) 
      RETURN SYS_REFCURSOR; 
END refcursor_pkg;
CREATE OR REPLACE PACKAGE BODY refcursor_pkg 
IS 
   FUNCTION filtered_species_cv (filter_in IN VARCHAR2) 
      RETURN endangered_species_t 
   IS 
      l_cursor_variable   endangered_species_t; 
   BEGIN 
      IF filter_in IS NULL 
      THEN 
         OPEN l_cursor_variable FOR SELECT * FROM endangered_species; 
      ELSE 
         OPEN l_cursor_variable FOR 
            SELECT * 
              FROM endangered_species 
             WHERE common_name LIKE filter_in; 
      END IF; 

      RETURN l_cursor_variable; 
   END filtered_species_cv; 

   FUNCTION data_from_any_query_cv (query_in IN VARCHAR2) 
      RETURN weak_t 
   IS 
      l_cursor_variable   weak_t; 
   BEGIN 
      OPEN l_cursor_variable FOR query_in; 

      RETURN l_cursor_variable; 
   END data_from_any_query_cv; 

   FUNCTION data_from_any_query_cv2 (query_in IN VARCHAR2) 
      RETURN SYS_REFCURSOR 
   IS 
      l_cursor_variable   SYS_REFCURSOR; 
   BEGIN 
      OPEN l_cursor_variable FOR query_in; 

      RETURN l_cursor_variable; 
   END data_from_any_query_cv2; 
END refcursor_pkg;
DECLARE 
   l_objects   refcursor_pkg.endangered_species_t; 
   l_object    endangered_species%ROWTYPE; 
BEGIN 
   l_objects := refcursor_pkg.filtered_species_cv ('%u%'); 

   LOOP 
      FETCH l_objects INTO l_object; 

      EXIT WHEN l_objects%NOTFOUND; 

      DBMS_OUTPUT.put_line (l_object.common_name); 
   END LOOP; 

   CLOSE l_objects; 
END;


Bottom line: Once you've got a cursor variable, you can use all the familiar features of explicit cursors with them: fetch, close, check cursor attribute values.

Cursor Variable Documentation

Cursor Expressions

A cursor expression, in essence, converts a subquery (SELECT statement) into a nested cursor (cursor variable).

Switching back to the HR schema, notice my use of CURSOR inside the all_in_one_cur. My INTO can then deposit the set return by CURSOR directly into a cursor variable defined in my PL/SQL block. I can then use standard cursor processing to fetch rows from that cursor variable.

CREATE OR REPLACE PROCEDURE cursor_expression_demo (location_id_in NUMBER) 
IS 
   TYPE refcursor IS REF CURSOR; 

   /* Notes on CURSOR expression: 

      1. The query returns only 2 columns, but the second column is 
         a cursor that lets us traverse a set of related information. 

      2. Queries in CURSOR expression that find no rows do NOT raise 
         NO_DATA_FOUND. 
   */ 
   CURSOR all_in_one_cur 
   IS 
      SELECT l.city, 
             CURSOR (SELECT d.department_name, 
                            CURSOR (SELECT e.last_name 
                                      FROM hr.employees e 
                                     WHERE e.department_id = d.department_id) 
                               AS ename 
                       FROM hr.departments d 
                      WHERE l.location_id = d.location_id) 
                AS dname 
        FROM hr.locations l 
       WHERE l.location_id = location_id_in; 

   department_cur   refcursor; 
   employee_cur     refcursor; 
   v_city           hr.locations.city%TYPE; 
   v_dname          hr.departments.department_name%TYPE; 
   v_ename          hr.employees.last_name%TYPE; 
BEGIN 
   OPEN all_in_one_cur; 

   LOOP 
      FETCH all_in_one_cur INTO v_city, department_cur; 

      EXIT WHEN all_in_one_cur%NOTFOUND; 

      -- Now I can loop through deartments and I do NOT need to 
      -- explicitly open that cursor. Oracle did it for me. 
      LOOP 
         FETCH department_cur INTO v_dname, employee_cur; 

         EXIT WHEN department_cur%NOTFOUND; 

         -- Now I can loop through employee for that department. 
         -- Again, I do need to open the cursor explicitly. 
         LOOP 
            FETCH employee_cur INTO v_ename; 

            EXIT WHEN employee_cur%NOTFOUND; 
            DBMS_OUTPUT.put_line (v_city || ' ' || v_dname || ' ' || v_ename); 
         END LOOP; 

         CLOSE employee_cur; 
      END LOOP; 

      CLOSE department_cur; 
   END LOOP; 

   CLOSE all_in_one_cur; 
END;
/

BEGIN
   cursor_expression_demo (1700);
END;
/


Cursor expressions are also commonly used in streaming and pipelined table functions.  Here's an example:

INSERT INTO tickertable
   SELECT *
     FROM TABLE (stockpivot (CURSOR (SELECT *
                                       FROM stocktable)))

Cursor Expression Documentation

DBMS_SQL Cursor Handle

Most dynamic SQL requirements can be met with EXECUTE IMMEDIATE (native dynamic SQL). Some of the more complicated scenarios, however, like method 4 dynamic SQL (variable number of elements in SELECT list and/or variable number of bind variables) are best implemented by DBMS_SQL. You allocate a cursor handle and then all subsequent operations reference that cursor handle. Here's an example:

CREATE OR REPLACE PROCEDURE show_common_names (table_in IN VARCHAR2)  
IS  
   l_cursor     PLS_INTEGER := DBMS_SQL.open_cursor ();  
   l_feedback   PLS_INTEGER;  
   l_name       endangered_species.common_name%TYPE;  
BEGIN  
   DBMS_SQL.parse (l_cursor,  
                   'select common_name from ' || table_in,  
                   DBMS_SQL.native);  

   DBMS_SQL.define_column (l_cursor, 1, 'a', 100);  

   l_feedback := DBMS_SQL.execute (l_cursor);  

   DBMS_OUTPUT.put_line ('Result=' || l_feedback);  

   LOOP  
      EXIT WHEN DBMS_SQL.fetch_rows (l_cursor) = 0;  
      DBMS_SQL.COLUMN_VALUE (l_cursor, 1, l_name);  
      DBMS_OUTPUT.put_line (l_name);  
   END LOOP;  

   DBMS_SQL.close_cursor (l_cursor);  
END;
/

BEGIN
   show_common_names ('ENDANGERED_SPECIES');
END;
/

DBMS_SQL Documentation 

Almost all code in this blog post may be accessed and executed on LiveSQL.

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Topics:
pl/sql ,cursors ,sql

Published at DZone with permission of Steven Feuerstein. See the original article here.

Opinions expressed by DZone contributors are their own.

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