Add generate json recipe

other-builtin-functions/08_generate_json_from_data/08_generate_json_from_data.md

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+# 08 Generate JSON from data
+
+![Twitter Badge](https://img.shields.io/badge/Flink%20Version-1.15%2B-lightgrey)
+
+> :bulb: This example will show how you can generate JSON using JSON functions.
+
+The source table (`clients`) is backed by the [`faker` connector](https://flink-packages.org/packages/flink-faker), which continuously generates rows in memory based on Data Faker expressions.
+
+There are many JSON functions in FlinkSQL. Within this recipe we are going to touch some of them: `JSON_ARRAY`, `JSON_OBJECT`, `JSON_OBJECTAGG`. You can find more information about these and other JSON functions in the [Flink SQL JSON Functions](https://nightlies.apache.org/flink/flink-docs-master/docs/dev/table/functions/systemfunctions/#json-functions).
+
+This table DDL creates a `clients` table. It contains names of a client and an array of addresses (2 elements) to which a company could send mail.
+
+## Script
+
+```sql
+CREATE TEMPORARY TABLE clients (
+ `prefix` STRING,
+ `first_name` STRING,
+ `last_name` STRING,
+ `address` ARRAY<STRING>
+)
+WITH (
+ 'connector' = 'faker',
+ 'fields.first_name.expression' = '#{Name.first_name}',
+ 'fields.last_name.expression' = '#{Name.last_name}',
+ 'fields.address.expression' = '#{Address.full_address}',
+ 'fields.address.length' = '3',
+ 'fields.prefix.expression' = '#{Name.prefix}'
+);
+```
+
+After creating the table we are going to use json functions.
+`JSON_OBJECT` allows to create an object by specifying keys and values
+`JSON_ARRAY` creates a JSON array.
+
+```sql
+SELECT 
+ JSON_OBJECT(KEY 'First Name' VALUE first_name, KEY 'Last Name' VALUE last_name, KEY 'address' VALUE address) AS `json_object`, 
+ JSON_ARRAY(first_name, last_name) AS `json_array` 
+FROM clients;
+```
+
+
+Furthermore, there are aggregating JSON functions 
+`JSON_OBJECTAGG` builds JSON object aggregating key-value into JSON object.
+The query will count prefixes and build json object for those which are appeared more than once.
+```sql
+SELECT JSON_OBJECTAGG(KEY prefix VALUE cnt)
+ FROM (SELECT prefix, COUNT(1) cnt FROM clients GROUP BY prefix) t WHERE cnt > 1;
+```
+
+and
+`JSON_ARRAYAGG` builds JSON array aggregating items into array.
+The query will construct a JSON array of distinct prefixes.
+```sql
+SELECT JSON_ARRAYAGG(prefix)
+ FROM (SELECT DISTINCT prefix FROM clients) t;
+```
+
+## Example Output
+![08_generate_json_from_data](08_generate_json_from_data.screen01.png)
+![08_generate_json_from_data](08_generate_json_from_data.screen02.png)
+![08_generate_json_from_data](08_generate_json_from_data.screen03.png)