# Group aggregators When grouping is active, each group row can show a rolled-up value per column - the sum of revenue, the average score, the count of deals. SvGrid does this declaratively: set `aggregate` on a column and the group header shows the result, formatted with that column's own `format`. > Live demo: Group aggregators - https://svgrid.com/demos/142-group-aggregators/ Every example on this page runs against these rows: ```svelte ``` ```svelte ``` ## Built-in reducers | `aggregate` | Result | | ----------------- | --------------------------------------------------- | | `'sum'` | Sum of the finite numeric values. | | `'avg'` | Mean of the finite numeric values. | | `'min'` / `'max'` | Smallest / largest value. | | `'count'` | Number of leaf rows in the group. | | `'countDistinct'` | Number of distinct values. | | `'extent'` | `"min – max"` range string. | | `'first'` | The first leaf row's value (e.g. a shared label). | Non-numeric and empty cells are ignored by the numeric reducers; an all-empty group yields no value (the header chip is hidden). ## Custom aggregators Pass a function for anything the built-ins don't cover - weighted average, median, percentile, distinct-with-rules. It receives the finite numeric values and the raw leaf rows: ```ts const median = (vals: number[]) => vals.length ? [...vals].sort((a, b) => a - b)[Math.floor(vals.length / 2)] : 0 const columns = [ { field: 'score', header: 'Median score', aggregate: median }, // weighted average using two columns off the raw rows: { field: 'rate', header: 'Blended rate', aggregate: (_vals, rows: Row[]) => { const w = rows.reduce((s, r) => s + r.weight, 0) return w ? rows.reduce((s, r) => s + r.rate * r.weight, 0) / w : 0 }, }, ] ``` ## The footer summary row The same reducers drive the grid's footer row, which totals the whole filtered set rather than a group. It is **off by default** - turn it on with `summary`: ```svelte ``` Out of the box each column falls back to a sensible guess: the sum of a numeric column, `Count: N` otherwise. Give a column its own `summary` to choose, using the reducer names from the table above: ```svelte ``` `summary` is independent of `aggregate`: one feeds the footer, the other feeds group rows, and a column can set both to different reducers. `enableRowSummaries` is the long-form name for the grid prop; `summary` wins when both are set. ## Notes - Aggregates roll up **all leaf rows** under a group, at every nesting level. - The aggregated value is stored on the group row, so `api.getDisplayedRows()` and `row.getCellValueByColumnId(id)` return it too - handy for exporting group totals. - The reducer is exported as `applyGroupAggregate(agg, columnId, rows)` for reuse outside the grid. See the live [Group aggregators](https://svgrid.com/demos/142-group-aggregators/) demo. --- SvGrid is the Svelte 5 data grid (`npm install @svgrid/grid`, MIT). This page: https://svgrid.com/docs/help/grouping/aggregators/ . All docs: https://svgrid.com/llms.txt