Deepak Prabhu B

Good Knowledge of about SQL and PL/SQL.
SQL provides an abstraction to access the RDBMS concrete implementation. The abstract concept that SQL tries to support is the relational model which in turn is supported theoretically by the tuple calculus.

It's purpose, in short words, is to provide a set of tools to the DBA or developer to define, query, control, and manipulate data in the relational model (tables, rows, columns, etc).

Part of its success is the declarative aspect. When you're querying in SQL, you don't care about execution plans, algorithms to fetch data, concurrency, parallelism, block sizes, pages, streams of bytes, etc. Providing abstraction from actual storage concerns made SQL immortal in my opinion.
Think about other stack layers, UI languages (JS) change every 6 months heavily influenced by major tech companies, backend languages change every couple of years (now mostly separating ops from application services logic), but relational databases have kept the same or similar syntax since around 25 years.
Of course, they are other alternatives like NoSQL, hierarchy languages and proprietary syntaxes but SQL is just a good example of simplicity and power of expression.
SQL concepts
What is RDBMS? RDBMS stands for Relational Database Management System. ...
What is a table? The data in an RDBMS is stored in database objects which are called as tables. ...
What is a field? ...
What is a Record or a Row? ...
What is a column? ...
What is a NULL value? ...
SQL Const