Add bbe for group by/collect

examples/aggregation/aggregation.bal

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+import ballerina/io;
+
+public function main() returns error? {
+ var orders = [
+ {orderId: 1, itemName: "A", price: 23.4, quantity: 2},
+ {orderId: 1, itemName: "A", price: 20.4, quantity: 1},
+ {orderId: 2, itemName: "B", price: 21.5, quantity: 3},
+ {orderId: 1, itemName: "B", price: 21.5, quantity: 3}
+ ];
+
+ var items = from var {orderId, itemName} in orders
+ // The `group by` clause create groups for each `orderId`.
+ // The `itemName` is a non-grouping key and it becomes a sequence variable.
+ group by orderId
+ select [itemName];
+
+ // List of items per `orderId`
+ io:println(items);
+
+ var quantities = from var {itemName, quantity} in orders
+ // The `group by` clause create groups for each `itemName`.
+ // The `quantity` is a non-grouping key and it becomes a sequence variable.
+ group by itemName
+ select {itemName, quantity: sum(quantity)};
+
+ // List of quantity per item
+ io:println(quantities);
+
+ var income = from var {price, quantity} in orders
+ let var totPrice = price*quantity
+ // The `collect` clause creates a single group and all variables become
+ // non-grouping keys
+ collect sum(totPrice);
+
+ // Total Income from orders
+ io:println(income);
+}

examples/aggregation/aggregation.md

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+# Aggregation
+
+The `group by` clause in the query expression can be used to group the elements in a collection. Grouping happens based on the grouping keys provided in `group by` clause. Non grouping keys becomes aggregated variables that can be used as a list or an argument to a rest parameter of a langlib function.
+
+The `collect` clause is used to collect the collection into one group. All the variables becoes aggregated variables and those variables can be used as a list or an argument to a rest parameter of a langlib function same as in `group by`.
+
+::: code aggregation.bal :::
+
+::: out aggregation.out :::
+
+## Related links
+- [Query expressions](/learn/by-example/query-expressions)
+- [Let clause in query expression](/learn/by-example/let-clause)
+- [Limit clause in query expression](/learn/by-example/limit-clause)
+- [Joining iterable objects using query](/learn/by-example/joining-iterable-objects)
+- [Querying tables](/learn/by-example/querying-tables)
+- [Create maps with query expression](/learn/by-example/create-maps-with-query)
+- [Create tables with query expression](/learn/by-example/create-tables-with-query)
+- [Create streams with query expression](/learn/by-example/create-streams-with-query)
+- [On conflict clause in query expression](/learn/by-example/on-conflict-clause)
+- [Nested query expressions](/learn/by-example/nested-query-expressions)

examples/aggregation/aggregation.metatags

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+description: This BBE demonstrates how to group a collection, how to handle non grouping keys.
+keywords: ballerina, ballerina by example, bbe, group, non-grouping keys, aggregate, collect

examples/aggregation/aggregation.out

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+$ bal run aggregation.bal
+[["itemA","itemA","itemB"],["itemB"]]
+[{"itemName":"itemA","quantity":3},{"itemName":"itemB","quantity":6}]
+196.2