grouping-aggregation.md

Grouping and Aggregation

LINQ may also be used to group query results and perform aggregation of data on the groupings. This grouping and aggregation is performed on the query server for improved performance.

:info: Note: LINQ group joins are not currently supported.

Simple Group

Grouping support is provided by the LINQ group clause. When grouping, you provide a key selector and an into clause. The key selector defines which property should be used to group.

The into clause provides a local name for the group to be referenced by for the rest of the query. When referencing this name, it is a list of the documents in a given group. It also has a .Key property, which is the value of the key selector for that group.

For more information about how the group clause works, see the MSDN group clause reference.

var context = new BucketContext(bucket);

var query = from beer in context.Query<Beer>()
            group beer by beer.BreweryId
            into g
            select g.Key;

await foreach (var doc in query.AsAsyncEnumerable()) {
    // do work
    // query will return the list of distinct brewery ids from all beers
    // Note: This form is shown for simplicity, using .Distinct() is probably a better solution
}

It is also possible to group by a combination of multiple columns.

var context = new BucketContext(bucket);

var query = from beer in context.Query<Beer>()
            group beer by new {beer.BreweryId, beer.Category}
            into g
            select new {g.Key.BreweryId, g.Key.Category};

await foreach (var doc in query.AsAsyncEnumerable()) {
    // do work
    // query will return the list of distinct brewery ids and categories from all beers
    // Note: This form is shown for simplicity, using .Distinct() is probably a better solution
}

Applying An Aggregate To A Group

LINQ supports six aggregates which can be applied to a group.

• Sum - Adds values
• Average - Averages values
• Min - Returns the lowest value
• Max - Returns the highest value
• Count - Returns the number of records
• LongCount - Returns the number of records as a 64-bit integer

Sum, Average, Min, and Max all accept a selector parameter that defines which property should be operated on.

var context = new BucketContext(bucket);

var query = from beer in context.Query<Beer>()
            group beer by beer.BreweryId
            into g
            select new {breweryId = g.Key, avgAbv = g.Average(p => p.Abv)};

await foreach (var doc in query.AsAsyncEnumerable()) {
    // do work
    // query will return the list of brewery ids and the average ABV from that brewery
}

Count and LongCount do not require a parameter.

var context = new BucketContext(bucket);

var query = from beer in context.Query<Beer>()
            group beer by beer.BreweryId
            into g
            select new {breweryId = g.Key, numBeers = g.Count()};

await foreach (var doc in query.AsAsyncEnumerable()) {
    // do work
    // query will return the list of brewery ids and the number of beers produced by that brewery
}

Ungrouped Aggregates

It is not required to use grouping to use an aggregate function. It is also possible to apply the aggregate directly to a query.

var context = new BucketContext(bucket);

var result = await (from beer in context.Query<Beer>()
                    select beer).LongCountAsync();

// result is the total number of beers in the bucket

var context = new BucketContext(bucket);

var result = await (from beer in context.Query<Beer>()
                    where beer.BreweryId = "21st_amendment_brewery_cafe"
                    select beer).AverageAsync(p => p.Abv);

// result is the average ABV of all beers made by 21st Amendment Brewery Cafe

Distinct Counts

Count aggregates also support the .Distinct() function. They return the number of discount results, rather than the total number of results.

var context = new BucketContext(bucket);

var result = await (from beer in context.Query<Beer>()
                    select beer.Name).Distinct().LongCountAsync();

// result is the total number of distinct beer names