Adding Nested Function Documentation

_search-plugins/sql/sql/nested.md

@@ -0,0 +1,211 @@
+---
+layout: default
+title: Nested Function
+parent: SQL
+grand_parent: SQL and PPL
+nav_order: 13
+redirect_from:
+- /search-plugins/sql/nested/
+---
+
+# Nested
+
+## Nested In SELECT Clause
+
+The nested function is used in the `SELECT` clause to unnest nested object type collections. The nested collection is flattened and a cartesian product is returned when querying against two or more nested collections. The nested function is supported in the V2 engine in the `SELECT` and `WHERE` clauses, see [query-processing-engines](https://opensearch.org/docs/latest/search-plugins/sql/limitation/#query-processing-engines) for more details.
+
+### Syntax
+
+The `field_expression` parameter is required and the `path_expression` parameter is optional. Dot notation is used to show nesting level for both `field_expression` and `path_expression` parameters. For example `nestedObj.innerFieldName` denotes a field nested one level. If the user does not provide the `path_expression` parameter, the value of the path will be generated dynamically. For example the field `user.office.cubicle` would dynamically generate the path `user.office`.
+
+```sql
+nested(field_expression | field_expression, path_expression)
+```
+
+### Using `*` With Nested In The SELECT Clause
+
+The `*` character can be used in the `nested` function `field_expression` parameter in the `SELECT` clause to select all inner fields to a nested object. For example a user could have a `field_expression` parameter of `nestedObj.*` to denote all inner fields under `nestedObj`.
+
+### Flattening
+
+Flattening is the process of changing the response format from OpenSearch by making the full path of an object the key, and the object it refers to the value.
+
+Sample Input:
+```json
+{
+  "comment": {
+    "data": "abc"
+  }
+}
+```
+
+Sample Output:
+```json
+[
+  { "comment.data": "abc" }
+]
+```
+
+### Query Example
+
+The following example uses a nested query in the `SELECT` clause:
+
+```sql
+SELECT nested(comment.data), nested(message.info) FROM nested_objects;
+```
+
+Dataset:
+```json
+{
+  "comment": {
+    "data": "abc"
+  },
+  "message": [
+    { "info": "letter1" },
+    { "info": "letter2" }
+  ]
+}
+```
+
+The results contain documents that match the nested query:
+
+| nested(comment.data) | nested(message.info)
+:----------------------| :---
+abc                    | letter1
+abc                    | letter2
+
+
+## Nested In WHERE Clause
+
+Nested object type documents can be filtered in a query by using the nested function in the `WHERE` clause of an SQL query.
+
+### Syntax
+
+There are two syntax options supported for the nested function when used in the `WHERE` clause of an SQL query. Both syntax options accomplish the same result of filtering a nested field with a literal value.
+
+#### Option #1
+
+The first option specifies the boolean condition inside the nested function with the `condition_expression` parameter.
+
+```sql
+nested(path_expression, condition_expression)
+```
+
+#### Option #2 (Supported in V2 Engine)
+
+The second syntax option uses the nested function on the left side of a predicate expression, and a `literal_expression` on the right. The `path_expression` parameter is optional and will be determined dynamically by the SQL plugin if not supplied. See [Nested Select Clause](#nested-in-select-clause) for a more in-depth description about the `path_expression` parameter.
+
+```sql
+nested(field_expression | field_expression, path_expression) Operator Literal_expression
+```
+
+### Query Example
+
+The following example uses nested queries in the `WHERE` clause and return the same result:
+
+```sql
+SELECT nested(message.info) FROM nested_objects WHERE nested(message.info) = 'letter2';
+SELECT nested(message.info) FROM nested_objects WHERE nested(message, message.info = 'letter2');
+```
+
+Dataset:
+```json
+{
+  "message": {
+    "message": [
+      { "info": "letter1" },
+      { "info": "letter2" }
+    ]
+  }
+}
+```
+
+The results contain documents that match the nested query:
+
+| nested(message.info) |
+:----------------------|
+letter2                |
+
+
+## Nested In ORDER BY Clause
+
+Sorting based on nested fields across documents can be accomplished by using the nested function in the `ORDER BY` clause of an SQL query. 
+
+### Syntax
+
+By default the `ORDER BY` will be in `ASC` order. The user can specify `ASC` or `DESC` after the nested function to specify an order in the query.
+
+```sql
+nested(field_expression | field_expression, path_expression)
+```
+
+### Query Example
+
+The following example uses nested queries in the `ORDER BY` clause:
+
+```sql
+SELECT nested(message.info) FROM nested_objects ORDER BY nested(message.info) DESC;
+```
+
+Dataset:
+```json
+{
+  "message": {
+    "message": [
+      { "info": "letter1" },
+      { "info": "letter2" }
+    ]
+  }
+}
+```
+
+The results contain documents that match the nested query:
+
+| nested(message.info) |
+:----------------------|
+letter2                |
+letter1                |
+
+
+## Nested In Aggregation Queries
+
+Nested fields can be aggregated by using the nested function in the GROUP BY clause and filtered in the HAVING clause of an SQL query. 
+
+### Syntax
+
+```sql
+nested(field_expression | field_expression, path_expression)
+```
+
+### Query Example
+
+The following example uses nested queries in the `GROUP BY` and `HAVING` clauses:
+
+```sql
+SELECT count(*) FROM nested_objects GROUP BY nested(message.info) HAVING count(*) > 1;
+```
+
+Dataset:
+```json
+{
+  {
+    "message": [
+      {"info": "letter1"},
+      {"info": "letter2"}
+    ]
+  },
+  {
+    "message": [
+      {"info": "letter1"},
+      {"info": "letter3"}
+    ]
+  }
+}
+```
+
+The results contain documents that match the nested query:
+
+| count(*) |
+:---------|
+2  |
+