Locking a row in a MYSQL table during a transaction

Question

I have a batch of queries like this:

START TRANSACTION
SELECT amount FROM piggybank WHERE id = 2
UPDATE piggybank SET amount = amount + 5 WHERE id = 1
COMMIT         


        

Answer 1

using a select ... for update query inside the transaction should give you the semantics you want - other updates will be locked, and other sessions attempting to take this same lock will block until you commit:

START TRANSACTION
SELECT * FROM piggybank WHERE id IN (1, 2) FOR UPDATE;
SELECT amount FROM piggybank WHERE id = 2;
UPDATE piggybank SET amount = amount + 5 WHERE id = 1;
COMMIT