Calculate balance per each category group - mysql

Question

I have table my_table which contains groups of categories, each category has initial budget (original_budget):
I am trying to add a new column balance so it contains the balance after reducing expense from the original_budget in each category group. Something like:
my try:
SELECT category, expense, original_budget, (original_budget-expense) AS balance
FROM my_table GROUP BY category order by `trans_date`
MySQL version: innodb_version 5.7.25
10.2.23-MariaDB

Answer 1

If you are using MySQL 8+, then it is fairly straightforward to use SUM here as a window function:
SELECT
trans_date,
category,
expense,
original_budget,
original_budget - SUM(expense) OVER
(PARTITION BY category
ORDER BY trans_date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) balance
FROM my_table
ORDER BY
category,
trans_date;
Demo
On earlier versions of MySQL, we can try to compute the rolling balance using a correlated subquery:
SELECT
trans_date,
category,
expense,
original_budget,
original_budget - (SELECT SUM(t2.expense) FROM my_table t2
WHERE t1.category = t2.category AND
t2.trans_date <= t1.trans_date) balance
FROM my_table t1
ORDER BY
category,
trans_date;
Demo

Answer 2

For All MySQL versions:
You can use MySQL User defined Variable to reduce balance amount for a category. For this keep same category records together with sorted dates.
SELECT
category,
expense,
original_budget,
IF(#cat <> category, #budg:= original_budget - expense, #budg:= #budg - expense) AS balance,
#cat:= category -- Set category to current value so we can compare it in next iteration
FROM my_table,
(SELECT #cat:= '' AS c, #budg:= NULL AS b) AS t
ORDER BY category, `trans_date`;
Output:
| category | expense | original_budget | balance | #cat:= category |
| A | 10 | 100 | 90 | A |
| A | 2 | 100 | 88 | A |
| A | 1 | 100 | 87 | A |
| B | 12 | 300 | 288 | B |
| B | 1 | 300 | 287 | B |
| B | 1 | 300 | 286 | B |
| B | 1 | 300 | 285 | B |