Which flavour of SQL backendless support?

I am having issues doing proper SQL query since I don’t know what kind of SQL version Backendless support.

There are several flavors of SQL: MySQL, MSSQL, postGreSQL, etc.

Which one is it?

Also in Codeless data API blocks wherever there is a table name can we add select operation there instead of single table name? if no can we have a custom SQL block like the custom JS code block ?

We support only the “where” part of an SQL statement, which is universally the same across most database systems. For details, please see the docs:
Search with the Where Clause - Backendless REST API Documentation
Search with SubQuery - Backendless REST API Documentation
Overview - Backendless REST API Documentation
Database Functions - Backendless REST API Documentation

Regards,
Mark

Thank you