Extend Metasql query builder section on DATA page

content/en/DATA.md

@@ -77,6 +77,27 @@ async ({ countryId }) => {
 };
 ```
 
+It uses Metarhia's [native SQL builder](https://github.com/metarhia/metasql) to access data from PostgreSQL database. Exactly that example demonstrates simple `select` usage, one of the builder's function among bunch of others available: 
+- `row` to query and receive data of a single row
+- `scalar` to get result as a single scalar value
+- `col` to get values from the one column in a form of array
+- `dict` to get results of two columns from table as a dictionary object where first column values becomes keys, second column  — its related values.
+- `count` to get number of records in some table optionally restricted by where filter
+- `insert`
+- `update`
+- `delete`
+- `returning`
+- `order`
+- `desc` — counterpart of `order` for descending ordering
+- `offset`
+- `limit`
+- `query` as approach to use pg driver directly for any parameterized SQL
+- `sql` as alternative approach to build query using special SQL template syntax (more on that later).
+
+In case you had started with [metarhia/Example](https://github.com/metarhia/Example) setup of the Metasql query builder is already done there. Otherwise it will be necessary to make few initial setup steps.  
+
+### Initial setup
+
 We need Postgres server running and dependencies installed: `npm i pg metasql`.
 To initialize connection to the database add following code to file `application/db/pg/start.js`:
 
@@ -181,15 +202,33 @@ INSERT INTO "City" ("name", "countryId") VALUES
 
 ## Metasql template sql
 
+Metasql has alternative syntax to create query: `sql` template function. Despite it looks like normal template string with SQL value, there is conversion to a parameterized query under the hood to fulfill security requirements. That syntax especially useful for complex queries when traditional builder's functions chaining approach becomes too complicated. Let's look at example:
+
 ```js
+const maxCount = 3;
 const query = db.pg.sql`
   SELECT * FROM "City"
-  WHERE "cityId" < ${5} AND "name" <> 'La Haye-en-Touraine'
+  WHERE "cityId" < ${5} AND "name" <> ${'excludedCity'}
   ORDER BY name
-  LIMIT 3
+  LIMIT ${maxCount}
 `;
 ```
 
+The result in `query` variable is a prepared statement, where:
+- number literal `5` converted into fixed query param
+- value of the `maxCount` variable included as fixed query param
+- `excludedCity` becomes a property name of an input argument, different value of which will become query parameter during every call to execute the statement.
+
+To execute this SQL query the statement object has special method `rows` . For example:
+
+```js
+const data = await query.rows({ excludedCity: 'La Haye-en-Touraine' });
+```
+
+> Note that if you don't provide value for the `excludedCity` — the key will be used among params as a string literal. 
+
+Other few methods of prepared statement quite the same as with traditional builder syntax: `row`, `scalar`, `col`, `count`, `dict`. That way you can create queries of unlimited complexity using available SQL operators.
+
 See more examples: https://github.com/metarhia/metasql/blob/master/test/sql.js
 
 ## Knex query builder