Php-mysql How to realize the money-shopping process (details about a lot of questions)

Php-mysql How to achieve buy things to deduct money flow (a lot of details)?
Assuming that the user spends money on the balance of 1 ' sugar ' This virtual commodity, the money balance and the number of users have ' sugar ' recorded on different tables, then we have to do is to deduct the user's money at the same time, the user's virtual inventory of the number of ' sugar ' +1. Suppose any user ' sugar ' is capped at 99

First, the current younger brother's ideas:

* Select to check the money balance on the user account (SQL statement 1)
* Compare money balances and a ' sugar ' price if the balance is sufficient then continue (PHP logic implementation)
* Transaction Start
*update the money balance and subtract 1 ' sugar ' from the price. Here's the question a: is the balance after the direct update to the ' sugar ' price (that is, PHP calculates the balance first), or does the MySQL do field subtraction? (SQL statement 2)
*select user ' sugar ' number (SQL statement 3)
* If the previous select has no results, then insert a record of the user's ' sugar ', the number of the purchase of ' sugar ', if the user's ' sugar ' record, then update ' sugar ' number, so that it plus 1. There is a problem here B: Because the maximum number of ' sugars ' for a single user is 99, I need to use PHP logic to determine if the ' sugar ' number +1 exceeds the upper limit of 99, but if there is no ' for update ' lock table in SQL statements 1 and 3, then combine issue a, once the user sends 2 buy ' Sugar ' request, it may appear that actually the number of ' sugars ' exceeds the upper limit of 99 (for example, reaching 100)
* Final Transaction submission

So there are 2 final questions:
C: Similar to the logic that consumes a resource, in exchange for another resource, must be ' select...for Update ' lock when a resource is select? (no first select and cannot make money enough or ' sugar ' whether the amount of the logical judgment, select after update and afraid of the Select to the data in the middle of the change)
D: Is there a way to simplify the above process?

Or there is no exhaustive article on this issue, if you can share it with gratitude.
------to solve the idea----------------------
This thing is guaranteed to be foolproof, it has to use the transaction, or the failure to rollback, the successful submission
------to solve the idea----------------------
Transaction processing must be used.
------to solve the idea----------------------
The transaction is first set, then the select for update takes out the current inventory, inserts a record into the log table if enough, and then update the inventory complete deduction, and finally releases the transaction.

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