CODEWITHSUNDEEP
mysqlloopsstored-procedures
kallakafar
kallakafar

Reputation: 735

looping through result rows and storing column data into stored procedure variables mysql

I m trying to learn stored procedure in mysql. I know the below example does not need SP, but this is ONLY for my learning purpose - just to understand how to loop through rows, and pick specific column data inside the row.

Assume that a SELECT on the table returns 5 records from my table with following structure -

table name: t1 columns: c1,c2,c3

i would like to loop through each of the 5 rows, and for each row, i would like to store the column cell data into SP variables varc1,varc2,varc3, so that varc1 = c1, varc2 = c2, varc3 = c3. i would be using these varc1,varc2,varc3 for further computation while within the loop.

i am assuming that a stored procedure is required, but i am not getting the approach in doing the row iteration and then picking data from each column in the row under consideration in the loop.

quite new to mysql/ stored procedure. thank you!

Upvotes: 3

Views: 32194

Answers (2)

Mr Purple
Mr Purple

Reputation: 2425

What your asking for, as you seem to know, is a particularly inefficient solution to most problems. However, since you specifically asked for it, and since (very occasionally) it's the only way to solve a problem (or the only way you personally have time to figure out how to solve it). Here's how you might do what you ask:

Create your t1 table eg:

CREATE TABLE t1(id INT PRIMARY KEY NOT NULL AUTO_INCREMENT, C1 INT, C2 INT, C3 INT);

Carry out your SELECT that you describe eg:

INSERT INTO  t1 SELECT NULL,c1,c2,c3 FROM the_table;

Now here's your Row-By-Agonising-Row "ReeBAR" low efficiency stored procedure:

Prepare space for your procedure and set the delimiter:

DROP PROCEDURE IF EXISTS ReeBAR;
DELIMITER ;;

Write your procedure:

CREATE PROCEDURE ReeBAR()
BEGIN
DECLARE n INT DEFAULT 0;
DECLARE i INT DEFAULT 0;
DECLARE varc1 INT DEFAULT 0;
DECLARE varc2 INT DEFAULT 0;
DECLARE varc3 INT DEFAULT 0;
SELECT COUNT(*) into n FROM t1;
set i=1;
WHILE i<=n DO 
  SELECT c1 FROM t1 WHERE id=i INTO varc1; 
  SELECT c2 FROM t1 WHERE id=i INTO varc2;
  SELECT c3 FROM t1 WHERE id=i INTO varc3;
  --queries which couldnt be made into set based queries go here---
  SET i = i + 1;
END WHILE;
End;

Alternatively if your id column in not sequential you can use the follwoing cursor form.

CREATE PROCEDURE ReeBAR()
BEGIN
  DECLARE cursor_ID INT;
  DECLARE cursor_VAL VARCHAR;
  DECLARE done INT DEFAULT FALSE;
  DECLARE cursor_i CURSOR FOR SELECT c1,c2,c3 FROM t1;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
  DECLARE varc1 INT DEFAULT 0;
  DECLARE varc2 INT DEFAULT 0;
  DECLARE varc3 INT DEFAULT 0;
  OPEN cursor_i;
  read_loop: LOOP
    FETCH cursor_i INTO varc1, varc2, varc3;
    IF done THEN
      LEAVE read_loop;
    END IF;
    --queries which couldnt be made into set based queries go here---
  END LOOP;
  CLOSE cursor_i;
END;

Don't forget to "end" the procedure you use and reset the delimiter

;;
DELIMITER ;

Finally to run your ReBAR procedure

CALL ReeBAR();

(code untested)

Upvotes: 6

FSP
FSP

Reputation: 4827

Use CURSORS and a loop to iterate over it. http://dev.mysql.com/doc/refman/5.0/en/cursors.html. Hope this helps!

Upvotes: 2

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