To calculate a running total in MySQL, use window functions in MySQL 8.0 or simulate with variables in older versions. In MySQL 8.0 , apply the SUM() function with an OVER() clause to compute the cumulative sum, optionally simplifying the window frame specification. For older versions, initialize and increment a session variable within a SELECT query, ensuring correct row ordering and handling group-specific resets when needed. Performance and complexity considerations favor window functions over variable-based approaches.
To calculate a running total in MySQL, you typically use window functions. If your version of MySQL supports them (MySQL 8.0 ), it's straightforward. For older versions, you’ll need to simulate the behavior using joins or variables.
Using Window Functions (MySQL 8.0 )
This is the cleanest and most efficient way. The SUM() function combined with an OVER() clause lets you compute a running total easily.
Example query:
SELECT
id,
amount,
SUM(amount) OVER (ORDER BY id ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS running_total
FROM sales;- This orders rows by
idand sums up theamountcolumn from the first row up to the current one. - The
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROWpart makes sure the window includes all previous rows plus the current one.
If you don’t need strict control over the window frame, you can simplify it:
SELECT
id,
amount,
SUM(amount) OVER (ORDER BY id) AS running_total
FROM sales;This gives the same result in most cases and is easier to read.
Simulating Running Totals in Older Versions (Before MySQL 8.0)
Older versions don’t support window functions, so you have to simulate this behavior using session variables.
Example query:
SELECT
id,
amount,
@running_total := @running_total amount AS running_total
FROM sales, (SELECT @running_total := 0) vars
ORDER BY id;- You initialize a variable
@running_totalto zero before the query starts. - As each row is processed, the
amountgets added to the variable, maintaining a cumulative total.
Some things to watch out for:
- Make sure the order is correct — running totals depend on the sequence of rows.
- Avoid relying on variable behavior across multiple queries — reset them explicitly if needed.
- Performance might be slower on large datasets compared to window functions.
When Grouping Is Involved
Sometimes you want a running total per group — like per customer or per category.
In MySQL 8.0 , you just add a PARTITION BY clause:
SELECT
customer_id,
order_date,
amount,
SUM(amount) OVER (PARTITION BY customer_id ORDER BY order_date) AS running_total
FROM orders;This will give a separate running total for each customer, ordered by their order date.
For older versions, you’d simulate this by resetting the running total when the group changes. That’s more complex and requires careful ordering and comparison logic.
That’s basically how you do it. Window functions are much cleaner and faster, but if you're stuck with an older version, using variables can work — just be careful with ordering and grouping.
The above is the detailed content of how to calculate a running total in mysql. For more information, please follow other related articles on the PHP Chinese website!
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