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Matouš Jan Fialka bc5c3c8bda [LIQ] Implement intra-aggregate order by (5: documentation)

Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>

2026-03-11 08:52:33 +01:00

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#maturity/experimental

The group by and having clauses of Space Lua/Lua Integrated Query support aggregate functions for grouped analysis, following SQL-style semantics.

After group by, each result row contains:

key: the group key (a single value or, for multi-key grouping, a table)
group: a Lua table containing all items in that group

All aggregate functions (such as count, sum, min, max, avg, and custom aggregates) can be applied in select and having clauses. Aggregate expressions are available in both forms: with or without a variable binding in the from clause. The variable _ always refers to the current item.

Field names used in group by are exposed as locals in having, select, and order by. Use #group to obtain the item count per group.

Note

Note The having clause acts only on grouped output. For filtering individual items, use where prior to grouping.

Examples

All example queries operate on tags.page, but will work with any query collection. As always, to see the underlying query, hover over the result table and click the Edit button to see the underlying query.

Counting with and without binding

Grouping pages by their first tag, and computing the count and aggregate statistics:

Without binding variable ${query [[ from tags.page group by tags[1] select { tag = key, total = count(name), min_size = min(size), max_size = max(size), avg_size = avg(size) } order by total desc ]]}

With binding variable ${query [[ from p = tags.page group by p.tags[1] select { tag = key, total = count(p.name), min_size = min(p.size), max_size = max(p.size), avg_size = avg(p.size) } order by total desc ]]}

Multi-key grouping and aggregate

${query[[ from p = tags.page group by p.tags[1], p.tags[2] select { first = key[1], second = key[2], count = count(p.name) } ]]}

Group filtering with `having` and aggregates

Only groups with more than two items and at least one tag set: ${query[[ from p = tags.page group by p.tags[1] having count(p.name) > 2 and key select { tag = key, total = count(p.name) } ]]}

Per-aggregate filtering with `filter(where ...)`

Individual aggregate expressions can include a filter(where <condition>) clause to restrict which rows contribute to that specific aggregate.

Unlike where (which filters rows before grouping) and having (which filters entire groups after aggregation), filter(where ...) applies per-aggregate, per-row within each group. Multiple aggregates in the same select can each have different filters.

${query [[ from p = index.tag 'page' group by p.tags[1] select { tag = key, total = count(p.name), big = count(p.name) filter(where p.size > 10), big_sz = sum(p.size) filter(where p.size > 10) } order by tag ]]}

The filter clause works with all aggregate functions: count, sum, min, max, avg, array_agg, and custom aggregates. When no rows match the filter condition, aggregates return their identity value: 0 for count and sum, nil for min, max, and avg, and an empty table {} for array_agg.

Intra-aggregate `order by`

Aggregate functions can include an order by clause inside the function call to control the order in which values are processed.

For commutative aggregates like sum, count, min, max, and avg, the intra-aggregate order by has no effect on the result because the value is the same regardless of iteration order. It is only meaningful for order-dependent aggregates like array_agg.

Basic example

Collect page names sorted alphabetically within each group:

${query [[ from p = index.tag 'page' group by p.tags[1] select { tag = key, names_asc = array_agg(p.name order by p.name asc), names_desc = array_agg(p.name order by p.name desc) } order by tag limit 5 ]]}

Combined with `filter(where ...)`

The order by and filter clauses can be used together. The filter is applied first (excluding rows), then the remaining rows are sorted before iteration:

${query [[ from p = index.tag 'page' group by p.tags[1] select { tag = key, big_by_size = array_agg(p.name order by p.size desc) filter(where p.size > 5) } order by tag limit 5 ]]}

Null handling

The nulls first and nulls last modifiers work inside intra-aggregate order by the same way they do in the query-level order by:

query [[
  from
    p = data
  group
    by p.category
  select {
    cat = key,
    items = array_agg(p.name
      order by
        p.priority asc nulls last
    )
  }
]]

Field access after grouping

Non-aggregated field references, such as name in select, refer to the first item in the group, matching common SQL and MySQL semantics.

${query [[ from p = tags.page group by p.tags[1] select { tag = key, first_page = p.name, n = count(p.name) } ]]}

Custom aggregators

Custom aggregator functions may be defined by the user using Library/Std/APIs/Aggregate.

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