Create a Cumulative Sum Column in MySQL

Question

I have a table that looks like this:

id   count
1    100
2    50
3    10

I want to add a new column called cumulative_sum, so the table wou

Answer 1

MySQL 8.0/MariaDB supports windowed SUM(col) OVER():

SELECT *, SUM(cnt) OVER(ORDER BY id) AS cumulative_sum
FROM tab;

Output:

┌─────┬──────┬────────────────┐
│ id  │ cnt  │ cumulative_sum │
├─────┼──────┼────────────────┤
│  1  │ 100  │            100 │
│  2  │  50  │            150 │
│  3  │  10  │            160 │
└─────┴──────┴────────────────┘

db<>fiddle

Answer 2

Using a correlated query:

---

  SELECT t.id,
         t.count,
         (SELECT SUM(x.count)
            FROM TABLE x
           WHERE x.id <= t.id) AS cumulative_sum
    FROM TABLE t
ORDER BY t.id

Using MySQL variables:

---

  SELECT t.id,
         t.count,
         @running_total := @running_total + t.count AS cumulative_sum
    FROM TABLE t
    JOIN (SELECT @running_total := 0) r
ORDER BY t.id

Note:

• The JOIN (SELECT @running_total := 0) r is a cross join, and allows for variable declaration without requiring a separate SET command.
• The table alias, r, is required by MySQL for any subquery/derived table/inline view

Caveats:

• MySQL specific; not portable to other databases
• The ORDER BY is important; it ensures the order matches the OP and can have larger implications for more complicated variable usage (IE: psuedo ROW_NUMBER/RANK functionality, which MySQL lacks)

Answer 3

You could also create a trigger that will calculate the sum before each insert

delimiter |

CREATE TRIGGER calCumluativeSum  BEFORE INSERT ON someTable
  FOR EACH ROW BEGIN

  SET cumulative_sum = (
     SELECT SUM(x.count)
        FROM someTable x
        WHERE x.id <= NEW.id
    )

    set  NEW.cumulative_sum = cumulative_sum;
  END;
|

I have not tested this