Update DYNAMIC Fields (meaning without mentioning the field names) with values from Other Tables through a Query in MySQL

Question

I am trying to update some fields, without mentioning the field name in the query, with values from other tables.

EXISTING_TABLE:

           +--------------------+
           |   Dynamic Columns  |
+---+------+------+------+------+
|ID |Item  |ABC   |GHQ   |XYZ   |
+---+------+------+------+------+
|1  |Item1 |100   |200   |Null  |
|2  |Item2 |Null  |Null  |600   |
+---+------+------+------+------+

With the query I am trying to formulate, the result of the table should be:

RESULT_TABLE:

           +-----------------+
           | Dynamic Columns |
+---+------+-----+-----+-----+
|ID |Item  |ABC  |GHQ  |XYZ  |
+---+------+-----+-----+-----+
|1  |Item1 |100  |200  |300  |
|2  |Item2 |400  |500  |600  |
+---+------+-----+-----+-----+

Those nulls should be replaced by the value of the source table below. In this table your will find how the fields above are dynamically produced.

SOURCE_TABLE:

+------+----------+------------+---------+
|ID    |Vendor    |Item        |Price    |
+------+----------+------------+---------+
|1     |ABC       |Item1       |100      |
|2     |ABC       |Item2       |400      |
|3     |GHQ       |Item1       |200      |
|4     |GHQ       |Item2       |500      |
|5     |XYZ       |Item1       |300      |
|6     |XYZ       |Item2       |600      |
+------+----------+------------+---------+

The tricky part here of this query is the columns are random or dynamic. It can only be determined at runtime. So, I should be using some syntax here to identify first the columns and update with values.

I am trying in this code but I could not find the expression for the for column_with_null_value and current_row_item:

CREATE DEFINER=`admin`@`%` PROCEDURE `null_update`()
BEGIN
    DECLARE n INT DEFAULT 0;
    DECLARE i INT DEFAULT 0;
    SELECT COUNT(*) FROM canvass_prices INTO n;
    SET i = 0;
    WHILE i < n DO
        UPDATE existing_table SET column_with_null_value = (SELECT price
           FROM source_table WHERE vendor = column_with_null_value
           AND item = current_row_item) 
           WHERE item = current_row_item;
        SET i = i + 1;
    END WHILE;
END;

UPDATE 1 Edited the entire question for clearer illustration of my case including the source table and my stored procedure attempt.

UPDATE 2 You may wonder why the data was not captured initially by the existing_table from source_table, it is because there were procedures needed to run to capture minimum prices in each item queried that leaves some columns remained null in the next item query.

UPDATE 3 Here's my actual script on how I arrived with the tables above.

This is the initial query for the first item:

DROP VIEW IF EXISTS min_prices;
CREATE VIEW min_prices AS
SELECT item_var.list_id, vendor.vendor_name, price
FROM price
JOIN item_var ON price.item_var_id = item_var.list_id
JOIN vendor ON price.vendor_id = vendor.list_id
JOIN item ON item_var.item_id = item.list_id
WHERE item_var.list_id = 1
ORDER BY price ASC
LIMIT 3;

SET @vendor1_name = (SELECT vendor_name FROM min_prices LIMIT 0, 1);
SET @vendor2_name = (SELECT vendor_name FROM min_prices LIMIT 1, 1);
SET @vendor3_name = (SELECT vendor_name FROM min_prices LIMIT 2, 1);

DROP TABLE IF EXISTS canvass_prices;
SET @canvass_var = CONCAT("CREATE TABLE canvass_prices AS
SELECT item.name, item_var.container, item_var.size, 
    (SELECT price FROM min_prices LIMIT 0, 1) AS '", @vendor1_name, "' ,   
    (SELECT price FROM min_prices LIMIT 1, 1) AS '", @vendor2_name, "' , 
    (SELECT price FROM min_prices LIMIT 2, 1) AS '", @vendor3_name, "' 
FROM item_var
JOIN item ON item_var.item_id = item.list_id
WHERE item_var.list_id = (SELECT DISTINCT list_id FROM min_prices)");
PREPARE canvass_statement FROM @canvass_var;
EXECUTE canvass_statement;
DEALLOCATE PREPARE canvass_statement;
SELECT * FROM canvass_prices;

And this is the second query for the succeeding items:

DROP VIEW IF EXISTS min_prices;
CREATE VIEW min_prices AS
SELECT item.name, item_var.container, item_var.size, vendor.vendor_name, price
FROM price
JOIN item_var ON price.item_var_id = item_var.list_id
JOIN vendor ON price.vendor_id = vendor.list_id
JOIN item ON item_var.item_id = item.list_id
WHERE item_var.list_id = 9
ORDER BY price ASC
LIMIT 3;

SET @vendor1_name = (SELECT vendor_name FROM min_prices LIMIT 0, 1);
SET @vendor2_name = (SELECT vendor_name FROM min_prices LIMIT 1, 1);
SET @vendor3_name = (SELECT vendor_name FROM min_prices LIMIT 2, 1);

call project_x.vendor1_name();
call project_x.vendor2_name();
call project_x.vendor3_name();

SET @canvass_var = CONCAT("INSERT INTO canvass_prices (name, container, size, 
                                    `", @vendor1_name, "`,
                                    `", @vendor2_name, "`,
                                    `", @vendor3_name, "`)
                           VALUES ((SELECT DISTINCT name FROM min_prices),
                                   (SELECT DISTINCT container FROM min_prices),
                                   (SELECT DISTINCT size FROM min_prices),                                   
                                   (SELECT price FROM min_prices LIMIT 0, 1),
                                   (SELECT price FROM min_prices LIMIT 1, 1),
                                   (SELECT price FROM min_prices LIMIT 2, 1))");
PREPARE canvass_statement FROM @canvass_var;
EXECUTE canvass_statement;
DEALLOCATE PREPARE canvass_statement;
SELECT DISTINCT * FROM canvass_prices;