Question

In MySQL, I am converting a table from a single row per item type (a quantity of items) to a single row per item, so that additional detail can be stored about individual items.

Here is an example source table:

id    parent_id    qty    item_type
--    ---------    ---    ---------
1     10291        2      widget
2     10292        4      thinger

I want to create a new table with a new column containing info that cannot be applied to more than one item. Thus, the above table would end up as follows:

id    parent_id    item_type    info
--    ---------    ---------    ----
1     10291        widget       [NULL]
2     10291        widget       [NULL]
3     10292        thinger      [NULL]
4     10292        thinger      [NULL]
5     10292        thinger      [NULL]
6     10292        thinger      [NULL]

Is there a way I can iterate or loop each row of the source table, inserting a number of records equal to the source qty column? I would prefer to do this in sql instead of code to keep all of the conversion steps together (there are many others).

Answer 1

Based on other answers which provided some insight, I was able to find additional information (by Kevin Bedell) to create a stored procedure and use a cursor in a loop. I have simplified my solution so that it matches the example in my question:

DROP PROCEDURE IF EXISTS proc_item_import;
DELIMITER $$
CREATE PROCEDURE proc_item_import()
BEGIN
    # Declare variables to read records from the cursor
    DECLARE parent_id_val INT(10) UNSIGNED;
    DECLARE item_type_val INT(10) UNSIGNED;
    DECLARE quantity_val INT(3);

    # Declare variables for cursor and loop control
    DECLARE no_more_rows BOOLEAN;
    DECLARE item_qty INT DEFAULT 0;

    # Declare the cursor
    DECLARE item_cur CURSOR FOR
        SELECT
            i.parent_id, i.qty, i.item_type
        FROM items i;

    # Declare handlers for exceptions
    DECLARE CONTINUE HANDLER FOR NOT FOUND
    SET no_more_rows = TRUE;

    # Open the cursor and loop through results
    OPEN item_cur;

    input_loop: LOOP

        FETCH item_cur
        INTO parent_id_val, item_type_val, quantity_val;

        # Break out of the loop if there were no records or all have been processed
        IF no_more_rows THEN
            CLOSE item_cur;
            LEAVE input_loop;
        END IF;

        SET item_qty = 0;

        qty_loop: LOOP

            INSERT INTO items_new
                (parent_id, item_type)
            SELECT
                parent_id_val, item_type_val;

            SET item_qty = item_qty + 1;

            IF item_qty >= quantity_val THEN
                LEAVE qty_loop;
            END IF;

        END LOOP qty_loop;

    END LOOP input_loop;
END$$
DELIMITER ;

Before asking this question, I had not used a stored procedures, cursors, or loops. That said, I have read and encountered them frequently on SE and elsewhere, and this was a good opportunity to learn

It may be worth noting that the example on Kevin’s page (linked above) does not use END%% (just END) which caused some headache in trying to get the script to work. When creating a procedure, it is necessary to change the delimiter temporarily so that semicolons terminate statements inside the procedure, but not the creation process of the procedure itself.