SQL Update: the row such that...? - mysql

Question

Suppose f is a string function such as concatenation by 0: f(0011)=00110.
Suppose mytable has columns string which is an index, and price.
I would like to do something like the following pseudo-code:
UPDATE mytable SET price(X)=0.5 FOR ALL THOSE ROWS X SUCH THAT
THE ROW Y SUCH THAT THE `string` OF Y equals f(the `string` FOR X)
HAS price(Y)=0
More generally, how can I reference
"the row such that [condition-holds-that-depends-on-the-current-row]"
in (My)SQL?

Answer 1

Setup demo
CREATE TABLE mytable
( id INT UNSIGNED NOT NULL PRIMARY KEY
, thestring VARCHAR(32)
, price DECIMAL(11,2)
) ENGINE=INNODB
;
INSERT INTO mytable (id, thestring, price) VALUES
( 1,'0011' , 123.45)
,( 2,'00110' , 45.67)
,( 3,'001100' , 0.00)
,( 4,'10' , 4.44)
,( 5,'100' , 5.55)
,( 6,'1000' , 0.00)
;
Write a query that identifies the Y rows in mytable
SELECT y.*
FROM `mytable` `y`
WHERE y.price = 0.0
ORDER BY y.id
Add a join to mytable to find the matching X rows
SELECT x.id AS x_id
, x.thestring AS x_thestring
, x.price AS x_price
, y.id AS y_id
, y.thestring AS y_thestring
, y.price AS y_price
FROM mytable `y`
JOIN mytable `x`
ON CONCAT(x.thestring,'0') = y.thestring
WHERE y.price = 0.0
ORDER BY y.id
Convert the SELECT into an UPDATE. (Replace SELECT ... FROM with UPDATE, and add a SET clause before the WHERE clause.
UPDATE mytable `y`
JOIN mytable `x`
ON CONCAT(x.thestring,'0') = y.thestring
SET x.price = 0.5
WHERE y.price = 0.0
Just replace the CONCAT(x.thestring,'0') with f(x.thestring).
Another option is to use a correlated subquery.
First, write a SELECT
SELECT x.*
FROM mytable `x`
WHERE EXISTS ( SELECT 1
FROM mytable `y`
WHERE y.thestring = f(x.thestring)
AND y.price = 0.00
)
And then convert that to an UPDATE. Replace SELECT ... FROM with the UPDATE keyword, and add a SET clause before the WHERE clause.
UPDATE mytable `x`
SET x.price = 0.5
WHERE EXISTS ( SELECT 1
FROM mytable `y`
WHERE y.thestring = f(x.thestring)
AND y.price = 0.00
)

Answer 2

Use an UPDATE with a self-join.
UPDATE mytable AS x
JOIN mytable AS y ON y.string = f(x.string)
SET x.price = 0.5
WHERE y.price = 0