Documentation Home
MySQL 5.5 Reference Manual
Related Documentation Download this Manual
PDF (US Ltr) - 27.1Mb
PDF (A4) - 27.2Mb
PDF (RPM) - 25.6Mb
HTML Download (TGZ) - 6.4Mb
HTML Download (Zip) - 6.5Mb
HTML Download (RPM) - 5.5Mb
Man Pages (TGZ) - 158.5Kb
Man Pages (Zip) - 262.0Kb
Info (Gzip) - 2.6Mb
Info (Zip) - 2.6Mb
Excerpts from this Manual

13.6.6 Cursors

MySQL supports cursors inside stored programs. The syntax is as in embedded SQL. Cursors have these properties:

  • Asensitive: The server may or may not make a copy of its result table

  • Read only: Not updatable

  • Nonscrollable: Can be traversed only in one direction and cannot skip rows

Cursor declarations must appear before handler declarations and after variable and condition declarations.

Example:

CREATE PROCEDURE curdemo()
BEGIN
  DECLARE done INT DEFAULT FALSE;
  DECLARE a CHAR(16);
  DECLARE b, c INT;
  DECLARE cur1 CURSOR FOR SELECT id,data FROM test.t1;
  DECLARE cur2 CURSOR FOR SELECT i FROM test.t2;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

  OPEN cur1;
  OPEN cur2;

  read_loop: LOOP
    FETCH cur1 INTO a, b;
    FETCH cur2 INTO c;
    IF done THEN
      LEAVE read_loop;
    END IF;
    IF b < c THEN
      INSERT INTO test.t3 VALUES (a,b);
    ELSE
      INSERT INTO test.t3 VALUES (a,c);
    END IF;
  END LOOP;

  CLOSE cur1;
  CLOSE cur2;
END;

User Comments
User comments in this section are, as the name implies, provided by MySQL users. The MySQL documentation team is not responsible for, nor do they endorse, any of the information provided here.
  Posted by Jérémi Lassausaie on February 3, 2015
Answer for Brent Roady :

I don't see any bug in the bahaviour described.

DECLARE a VARCHAR(255);
/* you declare a variable "a" without a specified default value, a=NULL */

DECLARE cur1 CURSOR FOR
SELECT a FROM table1;
/* You declare a cursor that selects "a" FROM a table */

OPEN cur1;
/* You execute your cursor query, a warning is raised because a is ambiguously defined but you don't see it */

FETCH cur1 INTO a;
/* you put your unique field in your unique row into a (basically you do "SET a=a;") so a is still NULL */

There is no bug report, just a misunderstanding.