Update query using select -- how does it know which row to update - mysql

Question

I have the following query that is working correctly but I don't understand why. I am changing the balance column for each row..How does it know which row to update for a particular customer.
UPDATE phppos_customers SET balance =
IFNULL((SELECT SUM(transaction_amount)
FROM `phppos_store_accounts`
WHERE deleted = 0 and customer_id = phppos_customers.person_id), 0);

Answer 1

For the sake of example, lets say you have the following two tables
[Users]
+---------+----------+
| user_id | username |
+---------+----------+
| 1 | patrick |
| 2 | chris |
+---------+----------+
[Names]
+---------+------------+-----------+
| user_id | first_name | last_name |
+---------+------------+-----------+
| 1 | Patrick | Stewart |
| 2 | Chris | Angel |
+---------+------------+-----------+
If you had a update query like the one in your original post, you would want to tell it how to align the two tables. If you had the clause WHERE Users.user_id = Names.user_id, you are effectively telling SQL to view the data as if both tables were aligned side by side, using the user_id in both tables to determine where they match up. This would mean the first_name and last_name in the [Names] table for user_id 1 will be what is used when updating the row in the [Users] table that is user_id 1. It is essentially viewing the data merged together, like this:
[Users] and [Names] tables aligned by the user_id columns
+---------+----------+---------+------------+-----------+
| user_id | username | user_id | first_name | last_name |
+---------+----------+---------+------------+-----------+
| 1 | patrick | 1 | Patrick | Stewart |
| 2 | chris | 2 | Chris | Angel |
+---------+----------+---------+------------+-----------+
So when SQL is doing the updating, it updates each row with the corresponding data from the other table, using this aligning to know which data to use for each rows update.
The process of aligning/merging data from multiple tables is called joining in SQL, here is some more information that illustrates how it works if you are interested.