> For the complete documentation index, see [llms.txt](https://crystal-garage.gitbook.io/lustra-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://crystal-garage.gitbook.io/lustra-docs/querying/the-collection-object/filter-the-query-1/aggregation.md).

# Aggregation

Collections expose common SQL aggregate helpers.

## Count

`count` returns the number of matching rows:

```crystal
user_count = User.query.count
active_count = User.query.where(active: true).count
```

You can choose the return type:

```crystal
count = User.query.count(Int32)
```

When the query contains `limit`, `offset`, or `group_by`, Lustra wraps the query in a subquery so the count applies to the filtered result set.

## Min, Max, Avg, and Sum

```crystal
max_id = User.query.max("id", Int64)
min_id = User.query.min("id", Int64)
average_id = User.query.avg("id", Float64)
total_ids = User.query.sum("id")
```

`min`, `max`, and `avg` require the expected return type. `sum` currently returns `Float64`.

`sum` does not sanitize the `field` input, so pass trusted SQL fragments only.

```crystal
total_post_ids = Post.query.where(published: true).sum("id")
# => Float64
```

For typed aggregate output, use `agg`.

## Custom Aggregates

Use `agg` for custom aggregate expressions:

```crystal
median_age = User.query.agg("PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY age)", Float64)
max_updated_at = User.query.agg("MAX(updated_at)", Time)
```

`agg` returns one scalar value. `agg` should not be used with `group_by` unless your intent is to wrap grouped rows first and aggregate that result as a whole. For grouped results, use `select`, `group_by`, and `fetch`/`pluck`.

`agg` also handles paginated/limited queries by wrapping them in a subquery.

## Exists

`exists?` checks whether at least one row exists:

```crystal
if User.query.where(active: true).exists?
  puts "There are active users"
end
```

`exists?` always queries the database. It is useful when you want a direct existence check instead of loading records.

`count` and `exists?` have different costs: `count` asks for row cardinality, while `exists?` only checks that at least one row matches.


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