You need to lock the row that you are reading so that you can update this row before anyone else can read it. Look at innodb-locking-reads, specifically for update

From docs

Locking of rows for update using SELECT FOR UPDATE only applies when autocommit is disabled (either by beginning transaction with START TRANSACTION or by setting autocommit to 0. If autocommit is enabled, the rows matching the specification are not locked.

Going off of your provided info, and not knowing more about NodeJS or your OS, Versions, setup etc. This is my thought process as what each issue I see.

Check The Basics

1. As silly as this may actually be, there are so many unknown factors.

   • I see you are running this on your own computer,
   • If you run this 10+ times in a row, is the behaviour the same?
     • If the end result is different, Check processes running close.
   • If possible,
     • please do your basic computer check (reboot, close other programs)
     • if possible, check it on another computer too

   After that is out of the way check:

Check saveBalance Function

1. Your saveBalance',async function Maybe the cause of the double SELECT...SELECT double INSERT...INSERT
   • My Gut tells me this is the issue based on the images, the order is wrong.

Check Database Settings

1. If the saveBalance',async function is fine. I would check your Database settings as based on your MySQL Console Logs & that you are using localhost

   • Your image shows SELECT...SELECT...INSERT...INSERT...
   • But it should be SELECT...INSERT...**SELECT...**INSERT...

2. This Might be due to an issue with your MySQL Database Settings.

   • Please check your Isolation Level. To Check this run this in your MySQL

SQL Code:

SELECT @@GLOBAL.TX_ISOLATION;

It also, might interest you to use START TRANSACTIONS;But be sure to enable multistatements in your config. and this is also directly tied to the TX_ISOLATION as well.

My example would modify your Query like so:

let SQL2="START TRANSACTION; INSERT INTO t_account(no,balance) VALUES('"+req.body.no+"',"+newBalance+"); COMMIT;";

Try Other Accepted Code as your own

Lastly, I also recommend reading 'Approach to multiple MySQL queries with Node.js - One should avoid the pyramid of doom'

I hope this helps.