How to update every each rows in different values in MySQL? - mysql

Question

id INT | food TEXT | memo TEXT
1 | Cucumbers | NULL
2 | Dandelions | NULL
3 | Salmons | NULL
3 | Cucumbers | NULL
4 | Tomatoes | NULL
Evening.
I have a table called results that splits each food values in several rows like the above.
A column named id refers to an individual animal, and a column food is an object what the animals would eat.
I would like to fill up a column memo by using REPLACE or UPDATE keywords like this:
3 | Salmons | Mostly liked it, only 15% didn't eat.
3 | Cucumbers | Only 10% have eaten, rest of all didn't even try.
A problem is MySQL always updates the same line like this:
3 | Salmons | Mostly liked it, only 15% didn't eat.
3 | Cucumbers | Mostly liked it, only 15% didn't eat.
This is my progress:
DROP FUNCTION IF EXISTS fx_length;
DROP FUNCTION IF EXISTS fx_splitter;
DROP FUNCTION IF EXISTS fx_split_row;
DROP PROCEDURE IF EXISTS App_forEach_memo;
DELIMITER //
CREATE FUNCTION fx_length(str TEXT, del VARCHAR(2), pos INT)
RETURNS INT
RETURN LENGTH(SUBSTRING_INDEX(str, del, pos - 1)) + 1 //
CREATE FUNCTION fx_splitter(str TEXT, del VARCHAR(2), pos INT)
RETURNS TEXT
RETURN SUBSTRING(SUBSTRING_INDEX(str, del, pos), fx_length(str, del, pos)) //
CREATE FUNCTION fx_split_row(str TEXT, del VARCHAR(2), pos INT)
RETURNS TEXT
BEGIN
DECLARE output TEXT;
SET output = REPLACE(fx_splitter(str, del, pos), del, '');
IF output = '' THEN SET output = NULL; END IF;
RETURN output;
END //
CREATE PROCEDURE App_forEach(IN target INT, strings TEXT)
BEGIN
DECLARE i INT DEFAULT 1;
REPEAT
UPDATE results
SET id = target, memo = fx_split_row(strings, '| ', i)
WHERE id = target;
SET i = i + 1;
UNTIL i = target
END REPEAT;
END //
DELIMITER ;
CALL App_forEach(3, "Mostly liked it, only 15% didn't eat.| Only 10% have eaten, rest of all didn't even try.");
I think I need to change this part SET id = target . . . for counting per same id numbers, but I don't know how to do this.
Are there any ways to update the memo values w/o creating an extra temp table?
Any tips, suggestions and answers for resolving this problem would be huge appreciated.
Thanks.

Answer 1

WITH cte AS ( SELECT food, ROW_NUMBER() OVER () rn
FROM results
WHERE #id = id )
UPDATE results, cte
SET results.memo = TRIM(SUBSTRING_INDEX(SUBSTRING_INDEX(#memo, '|', cte.rn), '|', -1))
WHERE results.food = cte.food
AND results.id = #id
AND cte.rn <= 1 + LENGTH(#memo) - LENGTH(REPLACE(#memo, '|', ''));
fiddle
You may put this query into a procedure if needed.
If you have ancient MySQL version which does not support CTE and window functions then emulate them in subquery using user-defined variables.